Make Quickly even quicker.

Added stuff to tests/Makefile to use the environment variable
THREADS (same as the one used in compiling GHC itself) to pass
to the new -j option to GHC. If THREADS is not set, defaults to 1.
Fix compilation problems on GHC 7.6, fixing #32.
Fix treatment of badPlus, backporting the "fail" fix to master.