Adds plan unit price and setup fee to pricing output

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Consolidates base prices on pricing output

- breaks plan price calculation into its own method
- fits better into Pricing Calculations procedures

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Adds plan base price support to pricing .attach

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Fixes addon base price attachment

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Merge pull request #150 from recurly/pricing-plan

Adds plan unit price and setup fee to pricing output