Add firebase database module about kotlinx.coroutines

[elizarov]

I really hope somebody contributes the corresponding integration. See this document on how to contribute: https://github.com/Kotlin/kotlinx.coroutines/blob/master/integration/README.md

[lucasvsme]

There is an implementation in this gist made by @beyondeye. Maybe a module can be written with that code.

Renaming some functions and changing their signatures to be extension functions that code would look like this.

@Test
fun `Can write and read from Firebase Database`() = runBlocking {
    val database = FirebaseDatabase.getInstance()
    val messageNode = database.getReference("message")

    messageNode.writeValue("Hello World")  // suspend function

    val message = messageNode.readValue(String::class.java) // suspend function

    assertEquals("Hello World", message)
}

There are other methods implemented, but I couldn't write tests for them yet. Also I'm not so familiar with coroutines yet.

[XinyueZ]

I think an upgrade to this topic should be transferred to this now.

https://firebase.google.com/docs/firestore/data-model

[lucasvsme]

@elizarov I've written suspend functions to Task and DatabaseReference here, but I don't know how to integrate the code with the existing Gradle build files of this project. Could you help with that?