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()`
Unfortunately this won't work well with the OSGi work that is progressing. The `Class.forName` will use the class loader of the `AddSensor` class, which is fine if there is a flat classpath. However, with OSGi then the entity's classes (including those for custom sensor types) could be in OSGi bundles that are added on-the-fly. This classloading with then fail.

However, this code will work for all the standard types like `String`, `URI`, etc so I'm inclined to include it. If @ahgittin agrees, then could you add some javadoc to this method like:

    /**
     * Creates a new sensor. The configuration can include the sensor {@code name} and {@code targetType}.
     * For the targetType, currently this only supports classes on the initial classpath 
     * (e.g. not those in OSGi bundles added at runtime).
     */
    @Beta