feat($filter): new date formats (timestamp, week in year, day in year)

Added new date formats to be used with date filter:
- 'u': Unix timestamp in milliseconds as used in javascript
- 'U': Unix timestamp (in seconds)
- 'W': Week in year
- 'D': Day in year
Merge branch 'new-date-formats' of github.com:florianorben/angular.js into new-date-formats