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()`
In the exception, can you include more information to help those diagnosing problems from log files. e.g. `"Invalid target type for sensor "+name+": "+className`