remove extraneous imports

As far as I can tell, in HTF 0.10, getArgs and exitWith no longer need to be explicitly imported.
use Cabal test suite feature
Merge pull request #19 from JPMoresmau/master

Use DiffOutput module from Diff library (>0.3.0)
Merge pull request #18 from brow/patch-1

Update the tutorial
clarify documentation on black-box-tests (issue #20)
support for JUnit-style XML output, suitable for Jenkins
(not tested with Jenkins yet)
support for parallel execution of tests
real support for parallel execution of tests
fixed two compile errors