Adding support for relative url redirects

[mcreale]

This PR adds support for relative URL redirects. if a relative url is found it will assume that we are using the current protocol and host name.

[samiahmedsiddiqui]

Thanks for the fix, @mcreale, and sorry it took so long to get back to you.

v2 is a complete rewrite, and lib/check-status-code.js no longer exists, so this PR can't be merged as it is. The relative redirect problem it addresses is fixed in v2 (#3). Every Location header is now resolved against the current URL, which covers /path, page, ../page and //host/path, and keeps non-default ports. The fix is covered by tests.

I'll close this once #3 is merged and v2.0.0 is released. Thanks again for reporting it and sending a fix!