Initial work on asset compilers
Copy from src to dest assets that don't require to be compiled
Made compiling process turned off by default
Raise an exception if an asset is missing during the compile time
Raise custom error in case of uknown asset engine
When compile a specific type only look for <filename>.js.*
Only compile/copy if destination is missing or source has been modified since the last time
Ensure to compile/copy by setting permissions to 0644
Build: Remove tmp/ directory only if present and make Travis CI :happy_cat:
Added concrete test with SASS (.sass & .scss)
Added concrete test with LESS
Drop LESS support for now.

This reverts a227f28
[ci skip] Document sources and preprocessors in README
[ci skip] minor README fixes
Default destination is "public/"
Introduced Configuration#root, it allows to use relative paths as sources for specific asset types
Let's lazily autocreate destination folder
Merge pull request #3 from lotus/assets-compilers

Assets preprocessors