Enable the engine to access global scope

This allows the engine to access global scope either via 'global' in node, or 'window' in the browser. This will be necessary for cfpb/hmda-pilot#21 due to the fact that after parsing the rules from the spec language into runnable code (*rules-as-code*) it will only have access to global scope. This allows us to set the HMDA file that is parsed into JSON (`_HMDA_JSON`) in the global scope so that the *rules-as-code* has access.

The engine itself becomes global as well, as `HMDAEngine`.