Allow to have more than one emulator started

* Starts the emulator specified in --avd option and runs tests in this emulator, even if other emulators or devices are connected
If --avd and --udid options are used simultaneously, --avd has preference.

* Print all errors while starting emulator
In some cases (like wrong snapshot), the emulator does not start but no error is thrown. 
The error message is in stdout, instead of stderr.

* Added option --avd-args to allow launch avd with additional arguments
Example: node . --avd AVD17 --avd-args="-no-snapshot-load -no-snapshot-save"

* Added option --avd-after-start-wait with the time in seconds that Appium will wait for the emulator before running the tests
It is necessary if the emulator takes a long time to start completely after 'device' state.
Without this wait, UIAutomator launch fails: "error: UiAutomator quit before it successfully launched"