Ensure everything is tested on travis

`cargo test` does not test dependencies
Fix warnings
Remove SubMesh type

This currently is not used for anything
Merge pull request #281 from bjz/submesh

Remove SubMesh type
Merge pull request #280 from bjz/tests

Ensure everything is tested on travis
Implemented prototype Light & Heavy batches in a separate module.
Implemented draw_batch
Implemented Batch trait for both light and heavy variants.
Ported the examples to use batches
Added docs, cleaned up the code