Add tax_code to plan, add on and adjustment

Used in tax calculations for VAT 2015 and
Avalara integration taxes.
Read and Write custom invoice notes from API
Version 2.3.7
add tax_region attribute to invoice and subscription
Adds CONTRIBUTING.md

- closes #123

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Adds AddOn#accounting_code

- closes #114

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>
Adding address to invoice and subscription preview

API feature is coming out that adds an address to the root level on
subscriptions and invoices for the preview path. Also adding the update
notes path.  This is part of a larger requirement for VAT 2015.

Testing
=======

For subscriptions:
  * grab an account
  * build a subscription preview
  * verify that the address is on there

```ruby
account = Recurly::Account.all.first
subscription = Recurly::Subscription.preview(plan_code: 'gold', account:
account)

puts subscription.address.inspect
```

For invoices:

  * add an uninvoiced charge to an account
  * grab that account
  * build an invoice off of it
  * verify that the address is on there

```ruby
account = Recurly::Account.all.first
invoice = account.build_invoice

puts invoice.address.inspect
```
Version 2.3.8
Use Travis CI containers