Fixes and enhancements to src/Gruntfile.js

I fixed the following issues:
- string path replacement does not work on Windows when paths are written with forward slashes
- .variables and .overrides are located in themes/ not definitions/
- grunt watch does not detect changes to .variables and .overrides

And added fast minification and packaging to the grunt watch task (only for modified files). I find this very useful because I usually only include the packaged CSS file in my project and this allows me to quickly make and test theme changes.

Please review. Thanks!