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
[nl] Chapter 02. Review, include new command output and add header.

   Reviewed translations and made them more consistent and in line with
   the recommendations of the Dutch language institute.
   Added a header to that effect to the file.
   Included the new modified command output.
accord avec "la plupart"

cf http://fr.wiktionary.org/wiki/plupart#Note sauf archaïsme, le verbe est au pluriel
Merge pull request #676 from JLuc/master

accord avec "la plupart"
[nl] Chapter 02. Review, include new command output and add header.

   Reviewed translations and made them more consistent and in line with
   the recommendations of the Dutch language institute.
   Added a header to that effect to the file.
   Included the new modified command output.
[nl] Chapter 02. More commands straightened

     On second viewing there were more commands to be corrected.
Merge branch 'chapter02' of github.com:Cpt-xx/progit into chapter02

Conflicts:
	nl/02-git-basics/01-chapter2.markdown
[nl] Chapter 02 Move the comment after first text.

    Aparently the Maruku test checks whether there is any HTML
    comment at the start (which is equally formed as the comment
    given in the English text concerning keeping the tabs unmodified.

    At this point it is not clear to me what this Maruku check consists
    of.
[nl] Chapter 02 Try to get past Maruku test
[nl] Chapter 01 Try to beat Maruku test
[nl] Chapter01 Add additional enter to avoid failing on Maruku test
[nl] Chapter 02 Try to pass Maruku test
[nl][en] Add request to revisit section and retranslate

    For the English document I added a request to revisit a section
    as comment.
    Retranslate Dutch and revisitted the Gitolite sections.
[nl] Chapter04 Start trying to beat Maruku test...
Merge pull request #675 from Cpt-xx/chapter02

[nl] Chapter 02. Review, include new command output and add header.
Merge pull request #678 from Cpt-xx/chapter04

[nl][en] Add request to revisit section and retranslate
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'