Allowable Mortgage Interest

This is a short hack I threw together in a couple days to show the difference between the Simple
and Exact Methods of computing allowable mortgage interest. It's not very polished because I know
very litle Javascript, but it should be adequate for the purposes of a demonstration. Briefly:

1. Equity is not considered.
2. Grandfather debt is not considered.
3. Calculations are afforded qualified loan limits of $1.1M or $550K for MFS.
4. Required fields are Average and Interest.
5. If Average is missing it will check Start and End to compute Average;
6. If either of those are missing it will check Days assuming the loan was taken
   out/paid off during the year to compute Average
7. If both are missing it will divide Interest by the Rate.

Minimal error checking is done. Garbage in yields garbage out.
Because I do not update Days it will only use a partial year if Days are less than 365.