Add BillingPlan model

The BillingPlan model stores some basic info about the plans available.

Also re-added the rake db:seed to set up an initial account (and more
importantly set up some example billing plans)
Added Billing Controller to handle payment processing

This adds support for processing payments through Chargify.

A Customer can now:

* Sign up for a paying plan
* Receive & view statements
* Change their plan (upgrade/downgrade/cancel)

All updates to their subscription (payment/nonpayment/plan change) will
be reflected in Helpful
Merge pull request #62 from drewblas/billing_controller

Billing controller