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