Improved byline gettext translations

Changes:
* Made translation a single gettext string with no html code in it. Instead all html is inserted as printf parameters. Cleaner gettext strings without loosing the hability to translate the full sentence as a whole. (Related: #608)
* Modified datetime attribute format from Y-m-j to Y-m-d, since day representation was lacking the leading zeroes.
* Made sure to get date format from options.
* Minor indentation fixes

These are the new byline strings:
* `Posted %1$s by %2$s`
* `Posted %1$s by %2$s <span class="amp">&</span> filed under %3$s.`