support nested `expose` declarations

See issue #34 

Here's a first attempt at supporting nested exposures.

**Note:**  Previously, any supplied block would be assigned to `options[:proc]` and called with the represented object and the runtime options.  I used that fact as a delineation, so...

1. If the supplied block has parameters, then it will be recognized as a Proc to determine the value of the exposure.
2. If the supplied block has no parameters, then it will be understood that the block contains nested exposures.

This will break any implementations that supply Procs with no parameters.