Createsend

* Added support for [single sign on](https://www.campaignmonitor.com/api/account/#single_sign_on) which allows initiation of external login sessions to Campaign Monitor.

* Added support for authenticating using OAuth. See the [README](README.mdauthenticating) for full usage instructions.
* Refactored authentication so that it is _always_ done at the instance level. This introduces some breaking changes, which are clearly explained below.
* Authentication is now _always_ done at the instance level.

So when you _previously_ would have authenticated using an API key as follows:

python
CreateSend.api_key = 'your_api_key'
cs = CreateSend()
clients = cs.clients()


If you want to authenticate using an API key, you should _now_ do this:

python
cs = CreateSend({'api_key': 'your_api_key'})
clients = cs.clients()


* Instances of any subclasses of `CreateSendBase` are now _always_ created by passing an `auth` hash as the first argument.

So for example, when you _previously_ would have called `Client()` like so:

python
CreateSend.api_key 'your api key'
cl = Client('your client id')


You _now_ call `CreateSend::Client.new` like so:

python
auth = {'api_key': 'your api key'}
cl = Client(auth, 'your client id')

* Created objects (clients, campaigns, lists, segments, and templates) now
retain identifiers they are given when created. This allows the following code
to be written:

python
client = Client()
client.create("Company Name", "(GMT+10:00) Canberra, Melbourne, Sydney",
"Australia")
details = client.details()


Previously, this code would have been written as follows:

python
client = Client()
client_id = client.create("Company Name",
"(GMT+10:00) Canberra, Melbourne, Sydney", "Australia")
client.client_id = client_id
details = client.details()

* Added support for including from name, from email, and reply to email in
drafts, scheduled, and sent campaigns.
* Added support for campaign text version urls.
* Added support for transferring credits to/from a client.
* Added support for getting account billing details as well as client credits.
* Made all date fields optional when getting paged results.

* Added the ability to set api_key for a CreateSend instance, rather than
only at the class level.

* Added Campaign.email_client_usage().
* Added support for ReadsEmailWith field on subscriber objects.
* Added support for retrieving unconfirmed subscribers for a list.
* Added support for suppressing email addresses.
* Added support for retrieving spam complaints for a campaign, as well as
adding SpamComplaints field to campaign summary output.
* Added VisibleInPreferenceCenter field to custom field output.
* Added support for setting preference center visibility when creating custom
fields.
* Added the ability to update a custom field name and preference visibility.
* Added documentation explaining that text_url may now be None or an empty
string when creating a campaign.