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()`
Interesting! I hadn't come across jsonPath before.

A few code comments:
* JsonPath.read(jsonString, element) will not necessarily return a `String` - e.g. if with test code `{"europe":{"uk":{"edinburgh":{"population":500000,"weather":"wet","lighting":"dark"}},"france":{"population":80000000}}}` then `$.europe.france.population` returns an `Integer`.
* `JsonPath.<String>read(jsonString, element)` returns null if it could not be found - this causes an NPE in the method `jsonParser.parse(...)`, rather than it getting to your nicer exception below.
* Would be good to add some unit test cases for this.

For test cases, I played around with the code below in `JsonFunctionsTest`:

    @Test
    public void testJsonPath() {
        JsonElement pop = JsonFunctions.getPath("$.europe.france.population").apply( europeMap() );
        Assert.assertEquals( (int)JsonFunctions.cast(Integer.class).apply(pop), 80*1000*1000 );
    }