DRY @groups for DSL

Now the default value is set in constructor
Refactor exceptions for assets_path

Matching a regexp was replaced with String#start_with? method to avoid assets_path
to be set like assets_path: 'js/foo/bar/assets'
No more necessity in config class variable for DSL
remove_components is set to true for bower:update and bower:update:prune to refresh bower.json file