Use custom API

DSTK allows users to [download a VM image](http://www.datasciencetoolkit.org/developerdocs#setup) of their entire service, allowing you to run faster repeated API calls (or run the service offline, on an intranet, or behind a firewall). 

In this update to your package, the URL for the DSTK API is now an R option instead of a hardcoded string. By default the option is set to the remote DSTK server. *Nothing changes for regular users of the plugin.*

If someone wants to use a different API server, they can set it programmatically with `options("RDSTK_api_base"="http://localhost:8080")`