L10n installer

fixes #1000 

This solution is bases on *Another solution: Native PO File support* as described at http://nsis.sourceforge.net/Creating_language_files_and_integrating_with_MUI

Translations are maintained on https://www.transifex.com/projects/p/owncloud/resource/mirall-wininstaller/

Job for transifex sync on Jenkins: https://ci.owncloud.org/view/translation-sync/job/translation-sync-mirall-wininstaller/

Here is a screenshot from the 1.5.0 installer in Portuguese (Brazil):
![bildschirmfoto vom 2013-12-04 00 30 22](https://f.cloud.github.com/assets/1005065/1669372/13716878-5c77-11e3-8100-73df206e059c.png)


@danimo @dragotin please review
@dschmidt FYI