Added support for getting an external session login URL.
Added support for the feature which allows an API caller to get a URL which initiates a login for an external Campaign Monitor session. API docs: http://www.campaignmonitor.com/api/account/#single_sign_on
Merge pull request #23 from campaignmonitor/add-external-session-support

Added support for getting an external session login URL.
Version 3.1.0.