Add ability to receive document ID when adding document

• Adds addAndProvideDocId (special thanks to @ofalvai for the contribution), useful if the document ID is needed when adding a document.

Dependency Updates

• Upgraded RxJava, Firestore, Kotlin, AGP and Gradle Wrapper

Add Set and Merge functionality when setting a Document

• Call RxFirestoreDb.setAndMerge() (Java) or documentReference.setAndMergeDoc() (Kotlin) to set a document with SetOptions.merge(), which will either set a new document or update an existing one by merging the fields.

Minor fixes

• Fix RxFirestoreKt set() extension function clashing with DocumentReference.set() by renaming it to setDoc() to follow pattern of other extension functions
• Update README to remove adding oakwoodsc bintray repo as we're now able to use jcenter

Fix publishing

• Added aar as an artifact when publishing. 1.0.2 was unusable, so migrations should occur from 1.0.1 directly to 1.03

Kotlin Support

Functional

• Adds Kotlin support for the already implemented API, future methods will be added for both Java and Kotlin
• Kotlin functions are simple extension functions that utilize the already-implemented Java methods

Non-Functional

• Updated bintray publishing strategy to use publications instead of configurations

Documentation

• Documented the API in RxFirestoreDB

Initial publish to Bintray

• Includes somewhat sloppy, un-documented support for some (but not all) Firestore APIs
• Future releases will improve the API greatly