Extract CubeDesc related code from CubeManager.java to CubeDescManager.java
Update test case data with the new metadata structure
This commit includes the following changes:

1. Code change for the metadata model structure change;
2. Update the example data, separating cube descriptor to cube + model descriptor;
3. Update some test cases.
Merge pull request #153 from shaofengshi/inverted-index

Update metadata structure and implement the CRUD for DataModelDesc