Exceptions for HTTP error codes

* Exceptions for generating 404, 401, 403 and 400 HTTP status
codes. These exceptions have been declared with the
"@ResponseStatus" annotation
Rivers controller wiring

* Fixed erroneous return types in the rivers controller
and implemented the handlers for handling river creation,
fetching, deletion and channel addition and deletion

* DTOs for the river, channel and channel options models

* Moved the set up for the controller tests to AbstractControllerTest
Refactor DAO tests

* Moved the DAO test classes to the
com.ushahidi.swiftriver.core.api.dao.impl package