RUBYAPI-6

Add support for parallel requests (#41). As we talked about, I left the implementation pretty bare bones.

Things to note:
* Requests must be made in the 'raw' format (ie SparkApi.client.[method])
* When in parallel, requests wont raise exceptions, you have to manually check the responses afterward. (https://github.com/ifightcrime/spark_api/pull/new/RUBYAPI-6#L2R34)