Native Emulators integration in CLI

For this PR the goal is to run the particular OS native emulator, using the default OS image in it. After that, we load and start the freshly built app in the running instance of the emulator.

Some peculiarities of the code: 
 - Android by default creates hosts of empty avds which we must sieve through and select a suitable one.
 - iOS requires ios-sim tool to be available in the PATH. Users are expected to downlaod and install this tool on their own.
 - Windows Phone 8 support has hardcoded "8.0" paths. They work for 8.1 WP SDK as well.

It should be `platform: wp8`