sends the default exposure to the override block with specified

Changes the default exposure access to a `call` API

As discussed, I changed the method of obtaining the
default exposure value to be lazily evaluated on
an object that is received by the block.

Includes specs and an update to the README
Uses a Proxy Object for the default for a custom block

The proxy object will lazily evaluate the default
if it is used in the custom block.
Merge pull request #91 from davidbiehl/proxy_object

Allow default exposure to be used in block