Add a lower version bound on tagged (due to 'witness' function)
Add more QuickCheck options and fix documentation

The following QuickCheck options were added:

  * QuickCheckReplay (--quickcheck-replay): Use a replay token to replay
  a previous test run. A replay token consists of a size and a seed.

  * QuickCheckMaxSize (--quickcheck-max-size): Constrain the size of the
  test cases quickcheck generates

  * QuickCheckMaxRatio (--quickcheck-max-ratio): Set the number of
    failing tests to accept per successful test before giving up

There is also a fix for a little documentation bug where it says "SmallCheck"
instead of "QuickCheck"

Fixes feuerbach/tasty#11
Add lower bound on stm. Fixes feuerbach/tasty#17
Print replay token & fix parsing bug

For some reason I forgot to add the displaying of the replay token ...
And there was a bug in the parsing of the token which caused it
to always fail.
Add lower bound on QuickCheck

maxDiscardRatio was renamed in QuickCheck 2.5 so we fail to compile on
2.4 and lower
QC: Change formatting of replay token
Don't let the main thread be garbage-collected

Fixes #15