fix pageChangePrevented method

while working on some new features for `form.coffee` in Shopify I discovered that the `page:before-change` event and the `pageChangePrevented` code is not consistent or working properly. `pageChangePrevented` was defined in `turbolinks.coffee` but never used and redefined in `click.coffee` with an underscore prepended. The code was actually used in the `click.coffee` code to fire the event and stop the request if the event returned false. This seems to stem from our refactor of turbolinks into separate files.

Changes
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* check pageChangePrevented in turbolinks#fetch
* now that pageChangePrevented is checked in fetch remove dead code from click.coffee
* add a proper test for pageChangePrevented
* send the turbolink url in the event.data (tested)

I checked the original turbolinks module and they have the same issue I noticed here - `pageChangePrevented` is only fired from the `click` code rather than checking inside of fetch. I can't see any reason for this - the only other thing that calls fetch is from the history handler and its working fine for me.

for review
@nsimmons @pushrax @qq99

I also bumped the version to 0.0.9 so we can ship this separate from the other things in the queue
this line looks so weird to me.
Isn't `triggerEvent` always returning `document.dispatchEvent event` which I assume is truthy?
perhaps I'm not seeing what's going on here