Add http proxy cababitlity for rest-trigger

[JetPlankl]

Pass Through Proxy

The idea is to have the REST Trigger and REST activity (https://github.com/project-flogo/contrib/tree/master/activity/rest)
changed in such away, that there is a possiblity to create a flogo app that has pass through proxy functionalities.

CHANGES TRIGGER:
Handle the existing functionality of create a path variable of the last part of the url coming in of the trigger (github.com/julienschmidt/httprouter):
example: incoming url: http://tibco.org/path/that/comes/in definition in trigger: /:restOfPath will
define a pathParams key: 'restOfPath' value: 'path/that/comes/in'
This :restOfPath definition in the trigger will also be exposed to the outside world e.g. mashery.
In mashery there is no option to change this mapping, therefore the trigger needs to be changed to automatically
create the 'restOfPath' variable for the rest of the path as defined in the trigger.
It will do so, using a boolean 'isPassThrough' to be able to turn this functionality on/off