Fix small typo in the Support docs
Fix API documentation issues

* 'Cookies' and 'Encodings' sections were not built because the
  reference to the functions was wrong.
* 'Exceptions' section had a wrong anchor link ('module-requests', same
   one as the main heading).
* Remove 'decode_gzip' function, which is no longer present.
Rewrite CaseInsensitiveDict to work correctly/sanely

Fixes #649 and #1329 by making Session.headers a CaseInsensitiveDict,
and fixing the implementation of CID. Credit for the brilliant idea
to map `lowercased_key -> (cased_key, mapped_value)` goes to
@gazpachoking, thanks a bunch.

Changes from original implementation of CaseInsensitiveDict:

1.  CID is rewritten as a subclass of `collections.MutableMapping`.
2.  CID remembers the case of the last-set key, but `__setitem__`
    and `__delitem__` will handle keys without respect to case.
3.  CID returns the key case as remembered for the `keys`, `items`,
    and `__iter__` methods.
4.  Query operations (`__getitem__` and `__contains__`) are done in
    a case-insensitive manner: `cid['foo']` and `cid['FOO']` will
    return the same value.
5.  The constructor as well as `update` and `__eq__` have undefined
    behavior when given multiple keys that have the same `lower()`.
6.  The new method `lower_items` is like `iteritems`, but keys are
    all lowercased.
7.  CID raises `KeyError` for `__getitem__` as normal dicts do. The
    old implementation returned
6.  The `__repr__` now makes it obvious that it's not a normal dict.

See PR #1333 for the discussions that lead up to this implementation
Remove Python 3.1 and 3.2 from the trove classifiers in setup.py
Merge pull request #1340 from ambv/tox_ini

Remove 3.1 and 3.2 from setup.py, add tox.ini to simplify testing on multiple versions
Merge pull request #1339 from cdunklau/case_insensitive_headers_cleanup

Rewrite CaseInsensitiveDict to work correctly/sanely
Merge pull request #1337 from amercader/master

Fix minor issues on the API docs
Merge pull request #1314 from duailibe/master

Fix small typo in the Support docs
Minor update to tests, added docstrings

Requested by @sigmavirus24.
Merge pull request #1341 from cdunklau/docstring_test_1329

Minor update to tests, added docstrings
Header keys should be native strings.

This commit follows a discussion on IRC. For more information, see the
Pull Request associated with it.