Show the publish date picker from within a table cell.

Gets rid of the popover and actionsheet previously used to show the picker.
Cleans up obsolete code.
Introduces a generic picker view for reuse in the future.
Fixes an issue where publish immediately would be overriden wtih a date.

Uses delegate methods instead of block callbacks.
Use NSIndexPath instead of an array of NSIntegers.
Merge pull request #1373 from wordpress-mobile/issue/1252-datepicker

Show the publish date picker from within a table cell.