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
 * [x] Check if it listens to webhook
 * [x] Fixup build (do not symlink `/tmp` directory anywhere)
 * [x] Fixup commit status in github
 * [x] Testing in development (due 15.02)