Mapping and mapping ref are mutually exclusive

DesignSpaceCfg.Builder checks that at most one of a MappingCfg
or a mapping reference has been set.
Added Spec suite and updated build file
Added a separate TestCleanup class
Added BasicDesignSpaceTest
Improved exception assertions

- Added two new assertions:
  - assertThrowsException
  - assertThrowsNothing
- Added a public utility method:
  - getException
- Fixed a bug in assertExceptionMatches
Split up UnitTest

Moved some exception-related tests to UnitExceptionTest.
Added javadoc comments