Adds EntityInitializers for http sensors and suport for full JSONPath

- Adds EntityInitializer for ´HttpRequestSensors´ by defining a ´HttpFeed´ over sensors.
- Full JSONPath support. Now is possible to specify a JSONPath (as defined here http://goessner.net/articles/JsonPath/ , thanks @ahgittin) by calling `HttpValueFunctions.jsonContentsFromPath()`
Don't worry about it here, but for future reference see `brooklyn.event.feed.http.HttpFeedTest`. This uses `BetterMockWebServer` to set up the HTTP responses so that tests can be run without an internet connection. Thus the test can be a unit test rather than an integration test.