add Str2Date func to handler date cell
Merge branch 'master' of https://github.com/lunny/xlsx into lunny-master
Rework date.go to be legible (as far as possible) and add tests.
Return NumberFormat string from Cell.
Test built-in numeric formats.
Merge branch 'lunny-master' with a lot of modification and testing.
Merge branch 'master' of github.com:tealeg/xlsx