Adds support for CC and BCC recipient types

Support for: to, cc and bcc recipient types as per mandill api.

Recipient types are defined as internal enum 'Type' defined in Recipient class.

Added gson serializer and deserializer for the new enum Type - mandrill
expects recipient type as lower-case string, but in java enums members are
usually written uppercase.

Default recipient type is set to 'TO'.
Merge pull request #18 from reviewpro/master

Adds support for CC and BCC recipient types. Thanks soulcheck!