Get rid of page.emulate() / page.emulatedDevices() methods

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.

lib/EmulationManager.js

@@ -14,38 +14,7 @@
  * limitations under the License.
  */
 
-const DeviceDescriptors = require('./DeviceDescriptors');
-
 class EmulationManager {
-  /**
-   * @return {!Promise<!Array<string>>}
-   */
-  static deviceNames() {
-    return Promise.resolve(DeviceDescriptors.map(device => device.name));
-  }
-
-  /**
-   * @param {string} name
-   * @param {!Object=} options
-   * @return {!Page.Viewport}
-   */
-  static deviceViewport(name) {
-    const device = DeviceDescriptors.find(device => device.name === name);
-    if (!device)
-      throw new Error(`Unable to emulate ${name}, no such device metrics in the library.`);
-    return device.viewport;
-  }
-
-  /**
-   * @param {string} name
-   */
-  static deviceUserAgent(name) {
-    const device = DeviceDescriptors.find(device => device.name === name);
-    if (!device)
-      throw new Error(`Unable to emulate ${name}, no such device metrics in the library.`);
-    return device.userAgent;
-  }
-
   /**
    * @param {!Connection} client
    * @param {!Page.Viewport} viewport