Issue18 add member count

Fixes #18 

Adds cURL call to collect member count via `/api/v1/users/get_member_count` and assign to message payload as \*|MEMBER_COUNT|\*. The merge_var will be used in all Mandrill transactional email templates to keep the footer message up to date.

![image](https://cloud.githubusercontent.com/assets/2119264/5091372/6fc7acba-6f16-11e4-8d28-50873dc28776.png)

Nope, this endpoint serves one purpose so no need for a constant as this will be the only place it's called from. I would like to move this function "method" into it's own class someday so other consumers in the Message Broker system can access it. Would it be worth building something DoSomething specific that can be put into composer.json?