Remove cleaning assets by default with bower:clean before precompilation

Add a configurable option (clean_before_precompile) that invokes rake bower:clean task
Reason: Many bower components are missing critical entries in main, or act as collection of libraries, so no there's no main entry on purpose
Reference: https://github.com/rails-assets/rails-assets/issues/121#issuecomment-36356857
set default value for @tasks
resolve merge conflicts