Ensure `lender` is set in Loan

This is a weird ism because of Ruby and Rails and execute order,
we want to ensure the `lender` is set, so we need to do it this way.
The crux of it

Some attribute setters, the form changes, and a little sprinkle of other magic
around the place - voila
Merge pull request #4 from smathy/develop

Nested belongs_to