add a `now` helper #163
Usage:

{{now ["format"]}}
Format parameters is one of:

"full": full date format. For example: Tuesday, June 19, 2012
"long": long date format. For example: June 19, 2012
"medium": medium date format. For example: Jun 19, 2012
"short": short date format. For example: 6/19/12
"pattern": a date pattern.

Otherwise, the default formatter will be used.
add timezone to ```dateFormat``` and ```now``` helpers #164