Quick-post post-type setting

Adds a setting to let users specify what type of post should be created
using the 'quick post' feature of the calendar
Refactor post_type_to_create to quick_create_post_type, and depend on the default option which is auto-loaded if the option doesn't yet exist
Move get_all_post_types() outside of the calendar object, because it's generally useful elsewhere

Minor refactor of validation
Merge pull request #181 from Automattic/cal-post-type

Quick-post post-type setting