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()`
should have had a comment:  the idea was to uncomment it when we were supporting it.  previously the only subclass, `SshCommandSensor` only needed to support strings so this was unnecessary, and this commented out line (successfully I'd say!) indicated how to proceed when other types were wanted.

i wonder could we:
* make `targetType` optional, e.g. defaulting to either `String` or `Object`
* support convenience names `string`, `object`, `int`, `double`, to make it easier for folks to specify the type
* rework `SshCommandSensor` to make use of this field, sharing the type coercion routines with the new classes introduced here.  (alternatively, move the `SENSOR_TYPE` field to `HttpRequestSensor`; thinking being, if this is on the superclass, all subclasses should support it.)