Support for nullable custom_quota

Mandrill allows for the custom_quota property to be omitted, which will then instruct Mandrill to manage the
quota automatically. For this to work, the custom_quota property needed to become nullable and instruct JSON.Net
to ignore it when null.
Merge remote-tracking branch 'upstream/master'
Add methods for listing Sender Domains and Checking a single Sender Domain
Add signing_domain and tracking_domain to EmailMessage type
Merge pull request #45 from HelloKoan/master

Support for nullable custom_quota