chenhaodong / mew-iview

Date utility functions `prevMonth` and `nextMonth` change the source
date when using `src.set[Date|Month|FullYear]()`. This change creates a
copy of the date at entrance of function so we have a pure function.

Made some code refactor and added specs for these functions.

add basic DatePicker test

Time-Picker new feature: allow steps in lists

This ads the possibility to choose `steps` in the formatting of the
lists. So we can have ”5 in five minutes” or ”every 15 minutes” instead
of having always lists of 60 items/minutes.

fix #1534