Improve speed of _fixWidth

The benefit of this is fairly noticable on a web page that formats a couple hundred dates. It is logically the same, but instead of string concatentation, it uses arrays and at the end performs a join.
Calculate correct string length while padding new string
Fix copy paste mistakes that made it through tests
Add tests to verify the format trimming and padding work as expected
Only trim years, as trimming others would cause loss of meaning
Merge pull request #117 from beeftornado/fixWidth-perf

Improve speed of _fixWidth