adding support for describe layout rest api

-Added additional method of describeLayout to SObjectType object.
-Added describeLayout forcetk method
-Also updated the default API version to 28.0
-Added test for the SObjectType.describeLayout

The tests currently point to cs1 instance which is still on 182 release. We will need to either point the tests to an org with Summer 13 release or we will need to wait till the release before we merge these changes as otherwise tests will fail.