Niftier summation of total order price.

By utilizing built in methods in Ruby and the Rails framework itself,
one can sum the price of an order in a way that is in my opinion
sweeter, neater and more mind-boggling. Also, it reduces the line count
by a total of 6!
Merge pull request #17 from badeball/nifty-order-summation

Niftier summation of total order price