sign a promissory note

Summary:
1.  Added fields for lender/borrower signatures and added them to the attr_accessible whitelist
2.  Removed double flash message container in notes#show view
3.  Notes#show view: changed note sign form to load the @note instance and to submit just the signed_by_lender/borrower fields.
4.  Notes#show view: don't load sign form if signed_by_lender/borrower field is already filled in
5.  Added specs to test borrower and lender sign and to check whether note sign forms are not rendered if note is already signed