Fixes #24(search results), various small fixes

Post types in search results are now properly filtered according to the
current language - this way duplicating posts(translation posts) are not
displayed any more.
Updated the way rewrite rules are generated - rewrite rules for posts
and other default rewrite rules are now obtained through filters and
modified for each language accordingly.
Added a couple of more translatable slugs(for comments, feed, author and
attachment URL's)
Added a couple of filters for the plugin's default options, additional
language slugs