opal/sprockets: added a couple of options to Opal::Server and updated doc.

The `use_index` option allows you to tell opal-sprockets if you want to use its default index page or not.
So for example if you have a sinatra application and a config.ru like this:
```ruby
require 'bundler'
Bundler.require

require './server'
require './client'

run Rack::Cascade.new([OpalApp, SinatraApp])
```
you dont have to switch applications order, you just tell OpalServer you do not want its default index.

The `source_map` option is just a shortcut to use instead of `Opal::Processor.source_map_enabled`.

Updating doc I think I should also fixed opal#458