feat(dateFilter): Add 'EE' formatter for shorter day string

Also added corresponding documentation and tests. Some applications require
more concise calendars. This will also help with directives like UI Bootstap's
Datepicker [1], for example, which only allows you to specify the
"day-header-format" with these pre-defined formatters.

[1] http://angular-ui.github.io/bootstrap/#/datepicker