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()`
This looks really useful.

Over time, given its name of `HttpRequestSensor`, I can imagine us adding support for other things like xquery for XML content, or for comparing the http status code, etc.

Can you mark the class as `@Beta` to give us more scope for such extensions in the future.