Adding plural translations to ownCloud

This PR adds the possibility to use plural in translations.
Closes #3935

Help needed
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I'm not very familiar with javascript. Please have a look at the __FIXME__ -parts in js.js

How does it work?
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The new function OC_L10n::tp( $text_singular, $text_plural, $count, $vars ) uses the singular translation if $count is exactly 1 or the plural translation if $count is not 1.

This should be the same behaviour gettext shows.

In the translations __%n__ will be replaced with the content of $count.

Possible improvements
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The content of $count could be checked/sanitized.