Document api types wrt validation

Part 3 in a series of changes towards data validation.
Merge pull request #312 from thockin/valid4

Document api types wrt validation
Fixed the data race found in kubelet_test.go
Merge pull request #331 from dchen1107/cleanup

Fixed the data race found in kubelet_test.go
Change error printing for easier debugging
Fix interface{} in api/types.go; plumb through system.
Change kublet to serve podInfo instead of containerInfo. Plumb through system.