User defined parser

Last one! So, I added the ability to pass in the parser that you want so user's can specify XML or CSV instead of JSON. They still have to override the 'format' parameter because I didn't want to force a dependency on Nokogiri since I think most will use JSON.

I also added test cases for all my new code and a couple of the methods you had originally created. I wasn't sure exactly how to test the next record and filter_date and post methods since I don't really use the 'import' functionality of the RedcapAPI.

Thanks so much!