Adds auth

Fixes #12 

- Adds `authenicate()` 
- to provide authenicattion to `curlPOST()` calls.

Related:
https://github.com/DoSomething/mbc-user-import/pull/25
The method name is `curlPOSTauth()` which means the call is specifically looking for auth values. Depending on the method name may not be the "elegant" solution?