Added unittests for cases seems to work

>>units.runTests()
test___add__ (units.__UnitTestCase) ... ok
test___div__ (units.__UnitTestCase) ... ok
test___divmod__ (units.__UnitTestCase) ... ok
test___eq__ (units.__UnitTestCase) ... ok
test___float__ (units.__UnitTestCase) ... ok
test___ge__ (units.__UnitTestCase) ... ok
test___gt__ (units.__UnitTestCase) ... ok
test___le__ (units.__UnitTestCase) ... ok
test___lt__ (units.__UnitTestCase) ... ok
test___mod__ (units.__UnitTestCase) ... ok
test___mul__ (units.__UnitTestCase) ... ok
test___ne_ (units.__UnitTestCase) ... ok
test___pow__ (units.__UnitTestCase) ... ok
test___radd__ (units.__UnitTestCase) ... ok
test___rdiv__ (units.__UnitTestCase) ... ok
test___rdivmod__ (units.__UnitTestCase) ... ok
test___repr__ (units.__UnitTestCase) ... ok
test___rmod__ (units.__UnitTestCase) ... ok
test___rmul__ (units.__UnitTestCase) ... ok
test___rpow__ (units.__UnitTestCase) ... ok
test___rsub__ (units.__UnitTestCase) ... ok
test___rtruediv__ (units.__UnitTestCase) ... ok
test___sub__ (units.__UnitTestCase) ... ok
test___truediv__ (units.__UnitTestCase) ... ok
test_combine (units.__UnitTestCase) ... ok
test_compare (units.__UnitTestCase) ... ok
test_convert (units.__UnitTestCase) ... ok
test_getCompoundUnit (units.__UnitTestCase) ... ok
test_getUnit (units.__UnitTestCase) ... ok
test_invert (units.__UnitTestCase) ... ok
test_isCompound (units.__UnitTestCase) ... ok
test_setunits (units.__UnitTestCase) ... ok
test_unitCompare (units.__UnitTestCase) ... ok
test_unitParse (units.__UnitTestCase) ... ok

----------------------------------------------------------------------
Ran 34 tests in 0.051s

OK