Migrates distribution to Maven Central

Updates dependencies and build tools, and migrates from jCenter to Maven Central.

Fixes an iOS interop issue

• Fixes an issue where the iOS SDK rejected audio messages sent from the Android SDK.

Images, texting, location message features

This release separates the Android SDK into its own repository to support package managers, and adds new features:

• Send messages to individual users instead of the whole channel
• Image messages
• Text messages
• Location messages