date/strftime

started date/strftime implementation.

following the tokens present on [python](http://docs.python.org/3/library/datetime.html#strftime-strptime-behavior), [ruby](http://www.ruby-doc.org/core-2.0/Time.html#method-i-strftime) and [C](http://pubs.opengroup.org/onlinepubs/007908799/xsh/strftime.html)

still missing some stuff but basic features are working.. I also added an internal `date/i18n_` module to allow setting the locale globally but I guess we need another option to override the default locale as well (maybe 3rd argument).

see #78