Add harness_class argument to TAP::Harness::Env
Make prove respect environmental variables #28
Get rid of superfluous environmental logic
Add META files to .gitignore
Implement external rulesfile for TAP::Harness

This implements a feature to let authors provide test scheduler rules
via a testrules.yml file without needing to customize the Makefile.PL or
Build.PL invocation of TAP::Harness.
Bump version to 3.31