Pyppeteer

* Mark as alpha

* Gracefully terminate browser process
* `Request.method` and `Request.postData` return `None` if no data
* Change `Target.url` and `Target.type` to properties
* Change `Dialog.message` and `Dialog.defaultValue` to properties
* Fix: properly emit `Browser.targetChanged` events
* Fix: properly emit `Browser.targetDestroyed` events

* BugFix: Skip SIGHUP option on windows (windows does not support this signal)

* Read WS endpoint from web interface instead of stdout
* Pass environment variables of python process to chrome by default
* Do not limit size of websocket frames

* BugFix:
* `Keyboard.type`
* `Page.Events.Metrics`

* BugFix (33)