Implement merge_vars option for Mandrill as :recipient_vars

Provide another argument like the following example:
recipient_vars: [{'email@email.com' => {'USER_NAME' => 'some name', 'USER_INFO => 'some info'}}]
Allow preserve_recipients boolean argument

Defaults to true (Mandrill default)
Allow nil args for global merge variables
Merge pull request #11 from ernsheong/merge_vars

Add merge vars functionality for user-specific information