Added some spike code for the developer facing api

Spiked the developer facing api for the custom checks
Made some minor changes to the CustomChecks. Made the IServiceControlBackend, common enough so hearbeats, or any messages can be sent to ServiceControl. Still work in progress. Related to #58
Refactoring messages out of Endpoint plug in implementation. Completed IServiceControlBackend implementation.
Merge pull request #56 from Particular/customchecks

Added some spike code for the developer facing api