Add support for self hosted deployments

resolves #16

For self hosted deployments of gopkg, the `-domain` flag should be used to specify the domain you want to use. For example `go.mysite.com`.

Now your github repositories are `go get`-able with a custom domain. `go get go.mysite.com/user/pkg.v0`.

To make the import path more concise, you could specify your github username using the `-github` flag. The import path `go.mysite.com/pkg.v0` would resolve to `github.com/user/pkg` instead of `github.com/go-pkg/pkg`.
I would rename the flag `github` to `github-username` or `username`.