Merge pull request #39 from tadfisher/master

Allow overriding invalid date selection behavior
A possible fix to IllegalStateException: Cannot add header view to list -- setAdapter has already been called when trying to add a header or a footer.
This crash occured on a Samsung Galaxy Nexus running 4.2.2 which is completely normal.
I removed the setAdapter from the consturctor and moved it to the private initialize method and set the adapter only if the getAdapter method returns null.
This change enables the possibility to add header or footer to the CalendarPickerView.
Merge pull request #40 from sannyo/master

Fix IllegalStateException when trying to add a header or footer to the CalendarPickerView on Samsung Galaxy Nexus running Android 4.2.2
Replace CheckedTextView with CalendarCellView; add custom states for easy overriding by client applications