add support for GitHub Enterprise oauth
use github enterprise url for boxen repos
update omniauth-github dependency
generating github enterprise API urls from env var

currently the GITHUB_ENTERPRISE_URL is used inconsistently as either the base url or the API url.  these changes standardize on the base url form.
Merge pull request #2 from mdelagralfo/ghe

generating github enterprise API urls from env var
add GITHUB_ENTERPRISE_URL explanations to README
Merge branch 'master' of https://github.com/boxen/boxen-web into patch-1

Conflicts:
	README.md
Merge remote-tracking branch 'upstream/master' into patch-1

Conflicts:
	app/helpers/views/splash/script.rb
Merge branch 'master' into test

Conflicts:
	README.md
	app/helpers/views/splash/script.rb
Merge pull request #13 from j-wilkins/patch-1

Add Github Enterprise support