Turbolinks Support

Using a callback in `lib/gmaps4rails/js_builder.rb` adds support for the `turbolinks` gem. This should, thereby, solve Issue #320.

I'm sorry, I couldn't test if this breaks your tests, since the tests for the `master` branch are not green on my machine (before changing anything).

### What I've changed:

If you can't use this pull request, here's what I did:

In `lib/gmaps4rails/js_builder.rb`, in `create_js`, within `if load_map?`, add the following line:

```ruby
@js << "$(document).on('page:load',  function() { if( $('.gmaps4rails_map').length > 0 ) { Gmaps.loadMaps(); } });"
```

This will create a callback for turbolinks, which triggers `Gmaps.loadMaps()` when a page is loaded via turbolinks. The line also checks if there's a map displayed on the page. Because otherwise, if you first open a page with a map, and then go to another page without map, turbolink will still call this callback. 

### Edit: Include API in header

* In order to include the api in the html head, one may use the new `gmaps4rails_api_script_tags` helper method.
* Including the api in the header is turned on by:
 * a) including the `turbolinks` gem (then the constant `Turbolinks` is defined)
 * b) adding this configuration in an initializer: `YourApp::Application.config.gmaps4rails = { include_api_in_header: true }`

### Test this on your project

To test this modification on another project or your test app, add my fork to the `Gemfile` of the test project, temporarily:

```ruby
gem 'gmaps4rails', '>= 2.0.1', git: 'https://github.com/fiedl/Google-Maps-for-Rails.git'
```

Furthermore, add the `gmaps4rails_api_script_tags` to the html head, as shown in this gist: https://gist.github.com/fiedl/6600555