9-29-20: Added Video Player & Kotlin.coroutines APIs in Android SDK

for more information check out the release note: https://github.com/pixlee/android-sdk/releases/tag/1.4.0

• Kotlin is added: please note that Kotlin and Java can be used in a project. To differentiate Java classes and Kotlin classes, I added a keyword ktx to Kotlin classes' name
• UI components for a video player, written in Kotlin.
• API requests(Album, PDP, and Analytics) via Kotlin.coroutines: this is a similar way of Scala's Future + await in dealing with async tasks. So, there's no need to use callbacks(interface) anymore. The SDK users can just get the result from the API methods directly within a scope(Kotlin.coroutines' language feature). I added sample codes throughout the demo app.
• a new library project is added to implement VideoWidget: https://github.com/pixlee/AndroidJZVideo
• Documentation for UI components and Kotlin.coroutines APIs
• This Android SDK is now provided via a Gradle to our users. They now can use this by adding a Gradle dependency to their Gradle file in their Android projects. There is a guide for it in README.