lessphp support

a lessphp port to minify, i've taken somebody else's work and rebased it to current master

overall expression: a group with single .less file: worked

@mrclay: could you drop your guidelines how to improve and make this work as you would intend?

tasks:
- [x] lessphp support
- [x] support import statements (multiple file caching/monitoring)
- [x] Convert Minify_Source::lastModifiedTime to ->getModifiedTime() #114
- [x] getCache interface #120