enable parallel check of languages

Use parallel to spawn as many subprocesses as possible to accelerate
the tests.
Change Maruku check to fully functional mode

this is cleans up the code.
Force output of Rake messages to stderr
Fix concatenation of file contents with 2 CR

The markdown files may not end with a '\n'. This would lead to
concatenating the last line of a previous file with the first line of
the next one.
Merge branch 'parallel_test'