Minitest 5

@axskern @os97673

 - guard recorder is gone, guard-minitest already takes care of that
 - we are lazy -> having reporters in load path does not auto-use it -> require it + use!
 - bring back after/before_test + after/before_suite
 - bring back total count before running the tests
 - make most reporters cleaner / drier
 - all reporters work (also verified rubymine) and looks nice in gallery
 - TODO change namespace from MiniTest to Minitest (after merge ? <-> less diff)
 - removed the parallel length hack thing, did not find any use for it
 - reporters get their io/options when minitest loads the plugin
 - minitest tests have 4 states passed, skipped, error, failure and failure is only detectable by `t.failure && !t.error?`

I'd propose to merge this soon and bump the major release, there might be some small issues left, but I don't want to sit here and rebase this giant diff, having something that 99% works on minitest 5 beats having nothing, I'll stay around to fix bugs that are found :)