Gettext rename

Renaming app.pot to foreman.pot so I will be able to add a project to Transifex.

Also all languages are now autodetected so we do not need to maintain the list manually.

Foreman in production mode now does use MO files instead direct (slow) reading of PO files. This need a change in our installation instructions. Right after database migration (and before starting foreman up) we need to add one extra step:

    rake gettext:pack

Also I am adding a Makefile that offers the same, but with GNU gettext package. Also it offers "make check" to do a deep checkup of all translations for general issues. There is also a README with a link to our wiki page where I want to put more info about it.

And I am adding German translation which replaces 

https://github.com/theforeman/foreman/pull/494

because it was in the non-UTF8 encoding and gettext was not able to parse it.