Add datepicker i18n and ability to set its width

[PawelStroinski]

The i18n can be done with the two new attributes:

• :i18n internationalization map with optional keys :days and :months (both vectors of strings)
• :goog? [datepicker-dropdown only] use goog.i18n.DateTimeFormat instead of cljs_time.format for applying :format

The other new attributes:

• :width [datepicker-dropdown only] a CSS width style.
• :position-offset [datepicker-dropdown only] px horizontal offset of the popup

These can be seen in action in the new I18n demo.

[superstructor]

Thanks @PawelStroinski 👍 😄

Resolved some minor merge conflicts, tested via demos page and merged.