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()`
`String.class.getName()` and similar below would be cleaner than these magic strings -- but i'm not going to block this PR for that!  nice to have these conveniences.  might want to move them to a shared utils if we need them elsewhere.