Better URL parsing through science

[benbalter]

Use Addressable rather than URI.parse to parse URLs.

This provides better support for URL encoded, non-ut8, and other edge case URLs.

Fixes #110.

[gjtorikian]

Me like.

[benbalter]

@gjtorikian passing-ish tests. One, failure, which I think is unrelated:

1) Links test fails on redirects if not following
     Failure/Error: output.should match /External link https:\/\/help.github.com\/changing-author-info\/ failed: 301 No error/
       expected "spec/html/proofer/fixtures/links/linkWithRedirect.html: External link http://timclem.wordpress.com/2012/03/01/mind-the-end-of-your-line/ failed: 301 No error\n" to match /External link https:\/\/help.github.com\/changing-author-info\/ failed: 301 No error/
     # ./spec/html/proofer/links_spec.rb:45:in `block (2 levels) in <top (required)>'