Added custom default android packageId

Creating a plain chrome app will still set `packageId` to `com.your.company.HelloWorld`

    cca create helloWorld 
    grep com.your.company.HelloWorld helloWorld/www/manifest.mobile.json

Importing a chrome desktop app (eg. no `manifest.mobile.json`) will set `packageId` to `com.your.company.DesktopApp`

    cca create existingDesktopApp --link-to=/path/to/desktop-chrome-app 
    grep com.your.company.DesktopApp existingDesktopApp/www/manifest.mobile.json

Importing a chrome mobile app won't update `packageId` if it's different from `com.your.company.HelloWorld`

    cca create existingMobileApp --link-to=/path/to/mobile-chrome-app 
    grep com.your.company.MobileApp existingMobileApp/www/manifest.mobile.json

BUG=#213

![image](https://cloud.githubusercontent.com/assets/634478/3964492/007e33d4-278a-11e4-99ab-9e77a58fc014.png)