Fixes #114

The choice type determines if a string is in a list of possible choices (name from to the python optparse choice option). This would replace the current heavy usage of string with match(SELF, '^(a|b|c)$') with

    "attr" ? choice('a', 'b', 'c')

String as a basetype is the most common one (or even only one). makes no sense for boolean i think, and long already has long(1..4) which is something similar. so only float is not covered, which i think is acceptable.