fixed main menu removing duplicate
rbenv ruby version to 2.0.0-p353
Collapsible navbar
Invoice payments now possible

Added the ability to pay an unpaid invoice
An invoice is now paid when it has a payment date, the boolean ‘paid’ flag was removed
Merge branch 'development'
Rbenv ruby v.2.1

Signed-off-by: Tommaso Visconti <tommaso.visconti@gmail.com>