Add Support for Email Template Creation

The API supports creating email templates via a post against /emails,
but the gem does not currently support making this request.

This can be made easier by DRY-ing up the ApiRequest class to have
generic post/get methods that support any generic endpoint/payload
combination.
Merge pull request #13 from seckenrode/add-support-for-email-template-creation

Add Support for Email Template Creation