Allow setting sync scripts at run-time

[heathkit]

Replace the setSynchronization API with something more sophisticated.

• Allow users to specify which client-side blocking scripts to run with each webdriver command.
• In addition to enabling/disabling scripts, allow changing the parameters (so Protractor can change rootEl)
• Allow configuring the timeout for each script.
• Potentially set a priority for the scripts, so they run in a defined order
• Allow providing a path to the client side scripts as a command line option

This is needed for angular/protractor#3857 and angular/protractor#3858.