implemented generation of qrcode for receiving payments by scanning it

+ shared view with shared helper
+ custom model with validation and methods for creating it
improved buttons
added validation and btc address validator
fixed enabled button without any wallets
fixed saving of wrong address
validations added
layout: added hr
more D.R.Y: introducing partials for various errors or hints
added centered class
renamed
imroved layout
improved layout and added "time ago"
added sender_addr to permitted params
moved to controller (seems to belong here?!)
closes #4
closes #3

+ added helper method for recent wallet address or label
+ cleaned up transaction new form
logo added
improved layout, styling and D.R.Y. regarding partials
set page_title everywhere; closes #6
added array cluster extension