Support for GitHub Enterprise about shipkit

[mockitoguy]

@mstachniuk, can you confirm suggested implementation above?

[mstachniuk]

I suggest to add 2 options to the configuration:

releasing {
    github.apiUrl = "https://github.in.your.company.com/api/v3"
    github.url = "https://github.in.your.company.com"
    // others ...
}

The first one to the API endpoint for generation API requests and second one for generating links for developers in pom (see: PomCustomizer.groovy)

[mockitoguy]

Nice proposal! I updated "api" -> "apiUrl". Let's make sure they are defaulted so that we don't have to configure it for typical OS builds.