Added top level describe metadata to provide a list of the SObjects available in the instance.

Also changed all json conversion from simple dict objects to OrderedDict - so that the ordering of keys in the returned Rest response is preserved.
Update Readme notes.
Allows support for Python versions newer than 2.4 but older then 2.7 / 3.x when using OrderedDict functionality.
Merge pull request #19 from malsmith/master

Added top level describe function and describe_layout function