Switched to NSDateComponents to create the date "short string"

This change also allows for localization of the units of time single characters.  NSDateComponents seems to perform well enough during scrolling - CALayer stuff swamped the CPU comparatively during instrumentation.
Made shortString a bit more translatable
Merge pull request #1428 from wordpress-mobile/issue/583-date-shortstring-i18n

Switched to NSDateComponents to create the date "short string"