Beaware argument requests from the constructor has been rename reqs

Add 'end' argument to PageList

this is sometimes useful; for instance I have a use case for
getting all pages in a given namespace that are alphabetically
between two specific points (because their names are in a
predictable format that involves alphabetically-sorted dates).
'end' is added as the last kwarg to avoid breaking any current
uses that rely on the kwarg ordering (thanks dan for pointing
that out).

I'll create a CREDITS.md file containing the info removed on this commit and more, so we have a comprehensive history document.

Use 'exec' in py3-compatible manner

[#106] fix GeneratorList with Python 3 (add __next__)

Credit for this fix goes to @tosher:
https://github.com/mwclient/mwclient/issues#issuecomment-154751657
I just figured out exactly what changes his comment suggested,
and poked around a bit to figure out what was going on and why
they're needed.

`GeneratorList` (and things deriving from it, e.g. `Category`)
was broken for iteration purposes in Python 3. In Python 3
the iterator protocol requires a `__next__` method, not a `next`
method as in Python 2. 8d0650cd renamed `List`'s `next` method
to `__next__` and added a small wrapper `next` method to account
for this, but did not make the same change for `GeneratorList`.
So when you iterate over a `GeneratorList` with Python 3, it
winds up calling `List.__next__` (since `GeneratorList` inherits
from `List`), not `GeneratorList.next`. So we don't hit the bit
that turns the results into mwclient page objects, and just get
the raw dicts from the `List` method.

We also have to change a couple of direct calls to `List.next`
to call `List.__next__` instead. If we don't do this, then when
the call originates from a child class, things get messed up,
because `List.next` calls `self.__next__` - and that will wind
up calling the child class' `__next__`, not `List`'s `__next__`.

Confused yet? :)

per https://docs.python.org/2/reference/simple_stmts.html ,
as exec is a function not a statement in py3, the py2 version
has been set to allow the subsequent statement to be a tuple,
so we can invoke it like this to make it both py2 and py3
compatible. Without this, byte-compiling the file fails under
py3.

Fix two typos

- Removing the need to use a double for loop to consume the the result
  by returning a list of revisions rather than a list of pages.
- Parsing the timestamps using the standard parse_timestamp method
- Adding tests

(#84)

Test in Python 3.5 too, now that it is out

Cache page text until next edit operation

Store the results of page.text() operations in a simple cache
dict. This avoids unnecessary remote roundtrips. Cache is
cleared on each successful page.save() operation. cache
argument can be set to 'False' to disable use of the cache.

Remove unused local vars

This attribute goes back to an older design, where
the 'edit page' workflow was different: the edit() method did
part of the work of defining what text you meant to edit. In
the current workflow, text() - which replaced the deprecated
edit() - is only for retrieving text, and save() is supposed
to do all the work of editing the page.

The 'section' attribute is therefore obsolete and dangerous.