Add a function to the base_type class for providing the option nafunc
Boilerplate for option[string]
Add a type pattern matching routine
Eliminate the separate errmode parameter in assignment, just use ectx
Add string to ?T assignment kernels so some NA tokens convert properly
Fix error in option type w.r.t. expression types
Some formatting changes
Change the set_utf8_string API on base_string

Will move this up to base_type as a common way to deal with strings
from types
Got the basics of the option[date/time/datetime] types working
Merge pull request #55 from mwiebe/more_option

More work on the option type