This patch:
- gets rid of `page.emulate` and `page.emulatedDevices`
methods. Instead, it is suggested to use `page.setViewport()`
and `page.setUserAgent()` methods.
- moves DeviceDescriptors to the top level of the puppeteer so that
it is convenient to require them.
- improves on documentation to describe the suggested emulation
approach.
References #88.
