Added entity initializers for HttpRequestSensors. Polling over a URI and retrieving a single JSON value from the response.
Add support for json-path (https://code.google.com/p/json-path) for full access to json response

Moved test to new package

Refactoring to avoid removing functionality:
now it supports both full path with JSONPath (through HttpValueFunctions.jsonContentsFromPath) and 1-level json element only.

rebased minor commits
Adds support for multiple type value on AddSensor.

Target type now needs to be specified as ConfigKey
Tests added for JsonFunctions.getPath
Target type made optional, default to String
Support for convenience type names.
This closes #48