Add endpoints for contacts

[mhenrixon]

Preferably split it up in one PR per entity. Have a look at the users endpoints and shared contexts for how to write an endpoint integration and responding tests.

• Contacts
• Automations
• Custom Fields
• Custom Field Values
• Tags

[timchaston]

PR for Custom Fields here: #60

I have another branch ready with Custom Field Values endpoints, but it builds off the work in the PR above so I'm waiting until that's accepted before I create a PR for that.

[timchaston]

PR for Custom Field Values here: #65

[timchaston]

PR for Tags here: #67

Another branch on the way for Contact Tags once Tags endpoints in general can be used for it.

[timchaston]

PR for Contact Tags here: #68

[timchaston]

PR for 'show' methods on Contact Tags here: #69