Rewrite build system
Drop non-descendant assets of source directory
Generalize extension matchers and simplify build
Generalize concept of manifest generators
Scope images directory the same way as javascripts and stylesheets
Generalize whole build
Separate writing files from creating transformations
Extract transformations code to separate method
Remove filesystem dependency of compute_transformation
Use only relative paths in compute_transformations
Extract transformer to separate class
Auto discover main asset if its basename is the same as gem name
Improve test suite, and perform some builder fixes