Fixed the "checkin" method

Looks like most the library is just for reading and/or GET routes.  I've added functionality for POST requests, all for the sake of getting the checkin method to work.  Other routes using POST will need to be updated.
* function param was misspelled
* turned "get" into "req"
* created new "get" and "post" functions
* post adds a "data" field to send post-body form data

I'll keep you updated on any other changes, eventually I'll revisit some of the OAUTH stuff, I thought lack of POST functionality warranted a pull request!

Thanks for starting/maintaining this library