support github enterprise

This Pull Request is to include the relevant changes to support the functionality in GitHub enterprise.

It includes an option page for the user to define the URL of her enterprise version.
I don't like much interpolating into regular expressions, I think the same can be accomplished in this case by having a simple url parser to extract a hostname and then use regular string comparisons.

    function host(url) {
        return url.trim().replace(/^(\w:\/\/)?([^\/?#]+).*$/, '$2').toLowerCase(); 
    }

    // Support a comma separated list of urls
    var urls = (gh_enterprise || 'github.com').split(',').map(host);

    if (-1 !== urls.indexOf(host(tab.url))) {
       ...