Invoices method assosation refactoring 
Fix Issue #27 [check if customer is an user customer]
Added Traslation
layout fix
Merge remote-tracking branch 'upstream/master'
replace Object.const_get with ActiveSupport constantize, added exception handling
moved 'invoice_per_year.rb' in app/lib
Merge pull request #53 from duncanita/master

association refactoring