Updated Client.prototype._exec to support sending data through request body rather than URI

To address issue #8
- Changed Client.prototype._exec to send data through the request body rather than the URI on POST and PUT requests
- Client.prototype._handleSuccess should now only attempt to parse JSON data returned if the response body variable was actually a string. (request returns a JSON object if you send one)

To address some of issue #7
- Some style changes (changed lines prefixed with ", " and added semicolons after variable declarations)