Changed before(:all) to before(:each) so we start with a clean state for each spec

See http://makandracards.com/makandra/11507-using-before-all-in-rspec-will-cause-you-lots-of-trouble-unless-you-know-what-you-are-doing
Added a config option for setting a redirect prefix that is added to every redirect URI
Added support for site.github.url and site.baseurl as the redirect prefix

Fixes #17
Added docs for prefix to README
Merge pull request #26 from akoeplinger/redirect-prefix