Clean up code in gfx_macro::mesh and make gfx_macro::mesh::decode_type more robust.
Improve documentation
Clean up formatting
Use quote macros to make #[vertex_format] implementation clearer
Improve project structure

`gfx_macros` is more consistent with the standard library's `regex_macros` crate.

The submodules in `gfx_macros` are now named after the attributes that they implement.
Use quote_expr! more extensively
Do not allow uint and int component types
Fix comment
Merge pull request #124 from bjz/macros

Assorted cleanups to macro implementations and project structure