Add tests/Tests.hs and integrate with cabal. 

Add failing (HUnit) tests for issues #1 and #4.

Does not use test-framework yet, but it should.

Also added a QuickCheck-based test, but didn't 
integrate, as it can produce false positives 
(e.g. when points are colinear).
Merge pull request #6 from nbogie/cabal_testsuite

Add tests/Tests.hs and integrate with cabal.