Adds auth

Fixes #12 

- Adds `authenicate()` 
- to provide authenicattion to `curlPOST()` calls.

Related:
https://github.com/DoSomething/mbc-user-import/pull/25
Not sure if it's a big deal or not, but it seems kinda weird that if you had it set, you'd also be sending along auth tokens for calls that didn't need auth. Eh maybe not a concern at all, just wanted to point it out.