adding support for describe layout rest api
Merge branch 'unstable20' of https://github.com/forcedotcom/SalesforceMobileSDK-Shared into unstable20
fixing the SObjectType bugs: 1) data shared among multiple instances. 2) prevent cache save to overwrite null data. 3) other small fixes.
Merge pull request #110 from akhileshgupta/unstable20

adding support for describe layout rest api