Updated chromedriver download methodology.

* Utilize S3 bucket XML instead of parsing HTML
* Encapsulate download URL logic in GoogleCodeParser
* Always re-fetch zip archive, because filenames all look the same
* Updated specs.
Plain HTTP for spec URLs
Merge branch 'download-via-bucket-xml' of https://github.com/mars/chromedriver-helper

Conflicts:
	lib/chromedriver2/helper.rb
	spec/google_code_parser_spec.rb