Node 6 support (#484)

This patch:
- introduces a transpiler which substitutes async/await logic with
generators.
- starts using the transpiler to generate a node6-compatible version of puppeteer
- introduces a runtime-check to decide which version of code to use

Fixes #316.

.gitignore

@@ -7,3 +7,4 @@
 *.pyc
 .vscode
 package-lock.json
+/node6