Only generate package formulae when styles and/or scripts could be found through bower.
Removed handling of empty resources as they are no longer available.
Refactored two almost identical methods into one to avoid code duplication.
Added missing braces for method-call.
Added changelog entry.
Moved allowed extensions to class-level property.
Refactored way option is passed to createSingleFormula()
Removed assertions for tests which are no longer needed due to the changed handling of package formulae.
Merge pull request #60 from daFish/feature/liststrict

Generate Assetic packages only when content for scripts and/or styles isn't empty