More Refactoring

[rylev]

This continues the refactoring of the cosmos crate to get it to a 0.1 release.

Highlights of these changes include:

• More documentation! The aim is to be able to use #[deny(missing_docs)]. We're making good progress towards this goal.
• ResourceType moves into the resources module.
• Create a headers module for all headers related functionality.
• Less implementation details are public (including ResourceType and the headers module).
• Move options like IndexingDirective and Query out of the top level module and into the appropriate resource modules

[rylev]

Here's how the top level docs are looking now 😊

For reference here's how it looks in master:

[MindFlavor]

Love it 👍