Amortization and housekeeping

Amortization implemented
 - Calculate monthly payment, determine total cost, and determine
   interest cost, normalized with respect to the price less rebate.
 - Newton root-finding algorithm used to solve amortization
   equation with respect to i (interest rate)
 - WARNING: Newton's Methid is not guaranteed to converge on a
	    root. Use bounded method if problems occur.
 - will require additional work to support variable compound rates
 - Asking for interest rate in terms of APR will automatically
   account for compound interest.

Output format modified
 - % sign for APR removed
 - if units belong in table cells, include it for dollar amounts
   as well, using string formatting syntax
 - string formatting syntax for dollar amounts to force exactly 2
   decimal places
 - calculated Dealership APR precision increased to up to 4
   decimal places

Input form modified
 - variable compound rates not required/supported with
   amortization implementation
 - removed dealer and bank compound period inputs
 - TODO: place upper limit on inputs to prevent overflows

Code
 - views.py heavily commented
 - variable names changed
   - e.g. interest_rate -> apr