Created 'Repo' simplified Api for Repository that only requires <generics> when you want it

See tests in molgenis-data-repo/src/test/java/org/molgenis/data/
- CsvRepo
- ExcelRepo
- JpaRepo (using Derby for test, output in molgenis-data-repo/target)
Also you can now use JpaRepository without code generation.

For this I had to 
- remove JpaEntity and hide ExcelEntity
- refactor much code to allow Entity.getEntityMetaData
- default ExcelRepository constructor

Next steps?
- remove generator for XyzJpaRepository?
- getEntityMetaData on ExcelEntity
- deprecated Entity.getLabelAttributeNames()