Docker requires DOCKER_HOST env to use format localhost:3000 without the scheme

This commit allows a URL to be used that does not contain a scheme.  If detected, the default scheme of `http://` is added.

Example:

Including the scheme in the environment causes Docker to fail
`$ DOCKER_HOST=http://localhost:4243 docker version
2014/08/06 13:02:29 Invalid bind address protocol: http://localhost:4243`

It works fine with just:
`$ DOCKER_HOST=localhost:4243 docker version`

Excluding the scheme currently with Docker API causes Docker API to fail
```
$ DOCKER_HOST=localhost:4243 bundle exec ruby -e "require './lib/docker'; puts Docker::Container.all.length"
/Users/matthew/.rbenv/versions/2.0.0-p481/lib/ruby/gems/2.0.0/gems/excon-0.39.3/lib/excon/connection.rb:88:in `initialize': no implicit conversion of nil into String (TypeError)
	from /Users/matthew/.rbenv/versions/2.0.0-p481/lib/ruby/gems/2.0.0/gems/excon-0.39.3/lib/excon.rb:123:in `new'
	from /Users/matthew/.rbenv/versions/2.0.0-p481/lib/ruby/gems/2.0.0/gems/excon-0.39.3/lib/excon.rb:123:in `new'
	from /Users/matthew/Projects/Ably/forks/docker-api/lib/docker/connection.rb:30:in `resource'
	from /Users/matthew/Projects/Ably/forks/docker-api/lib/docker/connection.rb:42:in `request'
	from /Users/matthew/Projects/Ably/forks/docker-api/lib/docker/connection.rb:57:in `block (2 levels) in <class:Connection>'
	from /Users/matthew/Projects/Ably/forks/docker-api/lib/docker/container.rb:187:in `all'
	from -e:1:in `<main>'
```

Now with this PR landed, you get the following
```
$ DOCKER_HOST=localhost:4243 bundle exec ruby -e "require './lib/docker'; puts Docker::Container.all.length"
# => 1
```