Improve error message when the asset pipeline is disabled

If you attempt to use xray-rails with an application that doesn't use the Rails asset pipeline (config.assets.enabled = false), you get an unhelpful error message:

```
/Users/jflanagan/code/xray-rails/lib/xray/engine.rb:15:in `block in <class:Engine>': undefined method `register_postprocessor' for nil:NilClass (NoMethodError)
	from /Users/jflanagan/.rvm/gems/ruby-1.9.3-p327-fast@pa/gems/railties-3.2.11/lib/rails/initializable.rb:30:in `instance_exec'
```

This PR attempts to provide a better experience:

```
/Users/jflanagan/code/xray-rails/lib/xray/engine.rb:93:in `ensure_asset_pipeline_enabled!': xray-rails requires the Rails asset pipeline. (RuntimeError)
The asset pipeline is currently disabled in this application.
Either convert your application to use the asset pipeline, or remove xrail-rails from your Gemfile.
	from /Users/jflanagan/code/xray-rails/lib/xray/engine.rb:10:in `block in <class:Engine>'
```