Obtain(componentKey,[default=value]) - finds/caches a needed component

Usage examples:

        class someClass(binding.Component):

            thingINeed = binding.Obtain("path/to/service")
            otherThing = binding.Obtain(IOtherThing)
            aProperty  = binding.Obtain(PropertyName('some.prop'), default=42)

someClass instances can then refer to their attributes, such as self.thingINeed, instead of repeatedly calling self.lookupComponent(someKey).

The initial argument to the Obtain constructor must be adaptable to binding.IComponentKey. If a default keyword argument is supplied, it will be used as the default in case the specified component key is not found.

XXX need to document IComponentKey translations somewhere... probably w/IComponentKey

__init__ (
        self,
        targetName,
        metadata=None,
        **kw,
        )

__repr__ ( self )

computeValue (
        self,
        obj,
        instanceDict,
        attrName,
        )