Pass the impromptu instance to the prompt class.
Make the cache method signature consistent.

Methods that the cache returns should accept a single callback, which
has the signature of `error, results`.
Pass the impromptu instance to the DB class.
Provide the prompt file with the Impromptu instance as context.
Convert the module registry into a class.
Merge pull request #15 from Impromptu/module-class

Begin to standardize classes; convert the module registry into a class.