Accept URI without scheme and host

Hi,

I think it's nice if we can omit URI scheme and host when describing http resource and Infrataster complements them automatically like this:

```ruby
Infrataster::Server.define(:proxy, '192.168.33.10')

describe server(:app) do
  describe http('/path/to/resource') do # accesses to http://192.168.33.10/path/to/resource
    # ...
  end
end
```

So, I wrote the implementation for the convenience above.

Additionally, after merging these patches, we can define default host name and make http resource use it instead of IP address:

```ruby
Infrataster::Server.define(
  :proxy,
  '192.168.33.10',
  http: {host: 'example.com'}
)

describe server(:app) do
  describe http('/path/to/resource') do # accesses to http://example.com/path/to/resource
    # ...
  end
end
```

Could you consider this pull request and merge it if you prefer it?

### Required consideration ###

Now scheme becomes http when omitted. Is it OK? Should we be able to set default scheme by `Infrataster::Server.define`?

Thanks.