Fix: Add checks from VIP's SVN pre-commit hook

Fixes #37.

- [x] Add YUI Compressor checks to Travis build as well
- [x] ~~Add Travis support for `upsearch`~~ (not needed since Travis runs in the root already)
- [x] Update README to indicate submodule usage.
- [x] Update README to indicate ability to copy `.travis.yml`
- [x] Update README to indicate new ENV vars
- [x] Stop searching up to find files when a ~~`.travis.yml`~~ `.git` is found (this will prevent plugins from running Core unit tests)
- [x] Rename project from `wp-plugin-dev-lib` to just `wp-dev-lib` since it's good for themes, plugins, and even entire sites.