precompile assets for heroku
Merge branch 'development'

* development:
  Refactor/cleanup: changed Invoice 'balance' method - removed class name.
updated 'filter' input label
Merge branch 'master' into development

* master:
  precompile assets for heroku
Invoices & Customers: removed column sorting logic from controllers.
Invoices & Customers: added sorting logic to views (and client-side js); added 'tablesorter' jQuery plug-in.
Customer: added 'customer balance' to list view.
Invoice: added 'billing-date' field that can be edited by user: db migration, model/view/controller updates.
Added a default date formatter for the app.
Added jQuery-UI library.
Invoice: billing date: added field to various views; applied 'datepicker' widget.
Added 'wicked pdf' and 'wkhtmltopdf' gems.
Invoices: added a pdf format (pdf files generated by wicked-pdf) for 'invoice' pages.
Fix: fixed 'profile' path/route in the main layout.
Merge branch 'development' into add-wicked-pdf