[cleanup] cleaning up tests
[task] finalized implementation #OSNG-191
[task] adding Extension to User #OSNG-191
[task] adding an extension adds the schema to the schema set
[enhancement] retrieval of single extension
[enhancement] retrieving all extensions returns an unmodifiable view
[cleanup] adding javadoc for new methods
[cleanup] cleaning up tests #OSNG-191
Merge pull request #3 from tkrille/OSNG-191

OSNG-191