Document most everything.

Some things are left undocumented, where they are uninteresting or REALLY
self-apparent. Adds `#[deny(missing_doc)]` to `render` and `device`, but also
some well-placed `#[allow(missing_doc)]`.

Closes #114
Address @kvark's review
Merge pull request #146 from cmr/doc-everything

Document most everything.
Make starting gfx with glfw cleaner.
Use builder for initialising gfx-rs