parse() blocks the node.js (or browser) event loop in case of extremely large zone files

Hi there! First of all, thanks for this library. 

I think the title basically describes the issue. Parsing gigantic zone files blocks the node.js event loop and thus it can't perform any other task while it's in progress. 

Example use-case: accepting HTTP requests while parsing a zone file is in progress. 

This issue is fixed by #38.