Lender borrower alternative

Here is another branch with the previous commits and the changes I suggested in the bounty.

    note has one lender
    note has one borrower
    lender belongs to note
    lender belongs to user
    borrower belongs to note
    borrower belongs to user

In the notes form the lender and borrower are loaded with a select field, this is just a temporary hack to make specs pass. Some brainstorming needs to be done with the whole notes creation process, maybe only the lender should be able to initiate the process and then the borrower could be invited to review the note and accept it ?