Inadequate documentation of glance architecture.

[Christopher MacGown]

20:01 <+soren> _0x44: Ok, so if I want to understand how Glance is supposed to fit into Nova. What do I need
               to read. I can read the code all I want, but it's hard to work out how it's meant to be used
               and fit into everything else.

If a core nova dev finds it difficult to figure out what glance is supposed to be/do, we need better documentation.

[Jay Pipes (jaypipes)]

Updated /doc/source/index.rst with a bunch of documentation on how to access Glance via API and via the glance.client.Client class. More to come, but should be good enough for a start...