allow note creator (the lender) to invite the borrower

Summary of changes:

1.  Adding devise and devise_invitable to User model, just using omniauthable for authentication
2.  Cleanup of old session methods 
3.  Send the borrower an invitation mail using the note borrower_email field
4.  Borrower accepting the invitation is redirected to dwolla authentication
5.  Show loans the borrower has been invited to sign in his profile page