Update _s.pot

Change "Posted on" string.
And add "by" string.
Related: #453, e24e53e
Merge pull request #507 from gatespace/update_pot

Update `_s.pot`.
Creates content-search.php and removes is_search() check from content.php
Merge pull request #512 from slobodan/content-search

First pass at creating a search-dedicated post template. Simplifies `content.php` and makes use of the template hierarchy.

Eventually we can think about removing more meta information, as appropriate.