Added a Makefile based on sax-rs and updated travis build.

- Updated gen/main.rs to account for changes in the latest sax-rs
- Added a -h/--help option to glrsgen
- Updated the binary created by the gen target to match the name in the README
- Ignoring the doc and test directories
- Added default search locations for linking: -L/usr/lib -L/usr/local/lib
Merge pull request #36 from photex/master

Added a Makefile and updated .gitignore to include a doc directory.
Use `extern crate`
Update compilation instructions
Merge pull request #37 from bjz/extern-crate

Use `extern crate`
Merge branch 'master' of https://github.com/bjz/gl-rs