Add limited test suite (using primarily QuickCheck)

I commented out the previous tests as they were broken (and I could
not find the time to fix them). I also switched to using Tasty, and
added the suite to the Cabal file (along with the -fhpc option).
Make Haddock documentation more coherent and fix three minor bugs

This commit fixes a couple of bugs as revealed by the recently added
test suite. The following bugs were fixed:

1. `xpAttribute' matched against a ContentText singleton, and failed
   to unpickle attributes with different text content.

2. `flattenContent' throwed an `UnresolvedEntityException' on
   comments.

3. `xpContent' could possibly return an XML comment.