Describe Layout calls

Adding a new method for accessing the describe layout of an object and
an overload allowing you to access the individual layouts given a record
type id. This would be very helpful for getting the picklists per record
type, but it seems like this hasn't been implemented by the Salesforce
team yet in their REST API.
Merge branch 'master' of github.com:brocsky/Force.com-Toolkit-for-NET into brocsky-master
Added unit tests for new methods
Merge branch 'brocsky-master'