Add tax_code to plan, add on and adjustment

Used in tax calculations for VAT 2015 and
Avalara integration taxes.
Merge pull request #160 from recurly/jh-digital_services_vat_2015_api

Add tax_code to plan, add on and adjustment
Read and Write custom invoice notes from API
Merge pull request #158 from recurly/use_invoice_settings_fields

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>
Merge pull request #164 from recurly/addon-accounting-code

Adds AddOn#accounting_code
Merge pull request #163 from recurly/tax-region

add `tax_region` attribute to invoice and subscription
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
```
Merge pull request #165 from recurly/adding_address_to_and_invoice_subscription_preview

Adding notes route and address to invoice and subscription preview
Version 2.3.8
Use Travis CI containers
Merge pull request #168 from recurly/travis

Use Travis CI containers
Merge pull request #167 from recurly/bump_2_3_8

Version 2.3.8
Threadsafe Recurly.api_key #159
fix for a case when code  running through Thread.new expects to find Recurly.api_key that was set in thr main thread #159