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()`
Is `<T extends String>` necessary? `String` is final, so nothing will ever extend it. You could use <T extends CharSequence>` or just remove the T? Or use a basic `Object` if it might be automatically converted at some point (not sure about that last option though)?