Extendable TraitType Tags and Validators

[rmorshea]

Depends on #335

Extend chained validators:

def less_than(n):
    def validator(trait, value):
        if value < n:
            return value
        else:
            raise TraitError("%s is not less than %s" % (value, n))
    return validator

def greater_than(n):
    def validator(trait, value):
        if value > n:
            return value
        else:
            raise TraitError("%s is not greater than %s" % (value, n))
    return validator

class A(HasTraits):
    i = Int(5).allows(less_than(10))

class B(A):
    i = Int(5, extends='allows').allows(greater_than(0))

a = A()
a.i = -5 # pass
a.i = 15 # raise

b = B()
b.i = -5 # raise
b.i = 15 # raise

Extend trait metadata:

class A(HasTraits):
    i = Int().tag(x=1)
class B(A):
    i = Int(extends='tags').tag(y=2)

B.i.metadata # {'x':1, 'y':2}

[rmorshea]

superceded by #429