GitHub Enterprise Support

This pull request allows specify custom GitHub host for [enterprise support](https://github.com/drone/drone/issues/17).

Roadmap:

 * [x] Introduce domain and api urls in settings (make sure you are using [patched](https://github.com/drone/go-github/pull/1) version of `go-github`)
 * [x] Create fields in account/admin/settings page
 * [x] Make it link account to enterprise github
 * [x] Fixed webhook url
 * [x] Fixed redirect from github_add page
 * [ ] Check if it listens to webhook
 * [ ] Fixup displaying of build

For now I'm stuck with last two steps. Any help will be appreciated.