Merge pull request #44 from HelloKoan/master

Added support for Subaccounts API
Update README.md
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'