Refactor third-party libraries usage and clean up karma config
* handle js-yaml and underscore libraries with bower (use libraries
downloaded with bower by sym-linking bower_components to libs dir in
* get rid of no longer used angular-filter;
* move libraries without official releases to custom-libs
* simplify file requirements in karma config.
Rewrite Merlin as a collection of Django apps for Horizon integration
Now a running Horizon instance is required to run Mistral Workbook
builder. Instructions on integrating Merlin and its extensions (the
first of which is Workbook Builder) are provided in README.md.