Clean up TODOs.

Also fix a test to explicitly pass.
Update README

The readme has been updated to represent the current state of
the project.

Closes #8
Travis Setup and Testing Tweaks

Travis setup has been added via a .travis.yml.  Note that
Travis runs nosetests directly through setup.py, without
tox.

Additionally, Python 3.4 has been added to the tox environment list,
and the test requirements have moved into test-requirements.txt
to facilitate use with Travis.
Mark low-level-api tests as requiring extensions

Previously, only the high-level API tests were marked as requiring
certain extensions.  This meant that otherwise good runs of low-level
tests would fail on systems that lacked one or more of the supported
extensions.

This commit moves the `_extension_test` decorator into a shared
test module so that it may be used by both the high-level and
low-level tests to indicate that a particular test should only
be run if an extension is present.
Fix Travis Cython Issues

This commit should fix issues caused by Cython compiling itself
on Travis by installing Cython as a pure-Python program.
Clean up the Sphinx configuration

This commit cleaned up the Sphinx configuration,
and changed all instances of PyGSSAPI to Python-GSSAPI.
Document utilites

This commit adds docstrings to the utilities module,
the __init__ file, and the exceptions module.

Part of #7
Document Credentials class

This commit adds documentation for the Credentials class.

Part of #7
Document Name class

This commit adds documentation for the Name class.

Part of #7
Document SecurityContext class

This commit adds documentation to the SecurityContext class.

Closes #7.
Document the ChannelBindings class

This commit documents the ChannelBindings class.

Part of #9
Document the OID class

This commit documents the OID class.

Part of #9
Document low-level credential store extensions

This commit documents both the credential store extensions
and RFC 5588.

Part of #9
Document low-level utilities

This commit documents lowl-level utilities and miscellanious
classes.

Part of #9
Document low-level credentials methods

This commit fills in missing documentation for low-level
credentials methods.

Closes #9
Change c_create_mech_list to c_create_oid_set

The function `c_create_mech_list` in the low-level converters
really just converted a OID set struct into a list of OIDs.
Ergo, it should be named as such.

Additionally, it makes more sense for the return value to be
a set, so make it as such.
Implement inquire_names_for_mech in low-level API

This commit implements the inquire_names_for_mech method
from RFC 2744, and adds a simple test as well.

Part of #10
Implement inquire_mechs_for_name in low-level API

This commit implements the inquire_mechs_for_name method
from RFC 2744, and adds a simple test as well.

Closes #10
Raise NotImplementedError on unavailable functionality

Closes #17
Allow input_cred to be None in add_cred_impersonate_name()

Closes #19