Added support for caching web requests

Hi Felipe - 
Awesome gem!  I added the ability to cache web requests (using VCR) so that when building and testing parsers for webpages, it's a bit faster (especially for large HTML pages) and so that you don't accidentally get yourself banned or throttled from sites that care about such things.

I hope you like the simple changes I included, basically just wrapping the mechanize and rest-client requests within a VCR block that caches to "fixtures/parser.yml".  I added an example in "examples/cache.rb" to show that it works.

Best,
Dustin