Add the Spanish translation (es_ES)
Add the domain parameter in the function _n in quark_posted_on()

Add the fourth parameter (the domain) in the _n function (http://codex.wordpress.org/Function_Reference/_n)
in the quark_posted_on() because if the _n function doesn't know the domain to retrieve the translated text
it can't show the translated number of comments.
Merge pull request #11 from amieiro/master

Add the Spanish translation (es_ES). Props @amieiro