Added HalSerializer that adds link helpers
Fixed webhook serializer test

1. Updated for the HAL output
2. Changed the expected to be a Hash.to_json.  Allows for line breaks in the code and makes it easier to read/type.  The conversion is very simple, so we shouldn't lose any test fidelity
3. Reversed expected/actual on the test so the output is readable when there's an error
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
Move doc rendering to the Apiary file in the root.
Updated docs for ENV

* Added explanations
* Moved the less important stuff in ENV to the bottom
* Outlined setup in README better and made more explicit reference to
editing the .env file
* Removed the '$' prompts from the setup steps so they can be
copy/pasted straight from the README.md
Standardize on Webhooks controller.

It's unlikely that there will be multiple IncomingEmails controllers, (hence the namespace) but I think there will be lots of controller's that handle Webhooks from other services.
Remove the seed rake task.

Same action can be done with the API now (and it stays updated) + Rails has it's own way of running seed data.
Standardize webhook URLs
Change Helpful contact point to the email endpoint.
Fixes WIP 164
Fixes BillingsController Test Issues
Make OAuth optional for API::MessagesController#create
Fix all remaining refs to Supportly.
Update Vagrant Config
Move OAuth Info to API Docs
Update Setup Instructions in README
Add bootstrap.sh for bootstraping a local mac dev setup
Vagrant - Configure Upstart
Remove bootstrap.sh (Mac OS Bootstrap Script)