Renamed `buf` to `command_buffer`

Closes https://github.com/gfx-rs/gfx-rs/issues/388
Merge pull request #389 from bvssvni/master

Renamed `buf` to `command_buffer`
Fixed update_buffer_vec bug

Closes https://github.com/gfx-rs/gfx-rs/issues/390
Use add_vec from add_struct
Merge pull request #392 from bvssvni/master

Fixed update_buffer_vec bug
Refactored shader parameter values to be mutable references to vectors.
Updated shader_param for the new interface, made all link fields to be options
Complete rewrite of the create_link deriving code, now much more secure.
Changed assertions to errors in upload_parameters