Add support for http file watch on Pegasus-EM triggering functionality.

Ryan's instructions:
1) Create a command object just like this one, then add it to commands here.
2) Add the route in views.py just like this one.
3) Add something like HTTP = "HTTP" to TriggerType here.
4) Create the trigger object in trigger.py to do the heavy lifting just like this.
In a nutshell, when you create a trigger (issue the CLI command), a request is sent to the service (running flask process). The service (in the route you’ve added), will create the trigger object with a READY state and add it to the database. The TriggerManager process will periodically polling that db and when it sees a READY trigger object, it will set the state to RUNNING, and start your custom trigger class, which should inherit from TriggerThread .