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)

The return value from the endpoint is a string, details here: https://github.com/DoSomething/dosomething/wiki/API#get-member-count