Add tests/Tests.hs and integrate with cabal. 

Add failing (HUnit) tests for issues #1 and #4.
Integrated with cabal test using exitcode-stdio.

Also added a properties-based test for #4 but didn't 
hook it up to cabal as it currently produces some 
false positives (e.g. when points are colinear).
Hopefully useful as a base for you.

Neither uses test-framework yet (as I had problems
 installing it), but that'd be desirable as the suite grows.