2.0.0

• Added customer login support
• Simplified checkout handling
• Improved support for error parsing
• Standardized API client interfaces (see BuyClient)
• Removed dependencies on deprecated APIs
• Updated sample app, including addition of Customer account sample code
• Added built in support for RxJava, and cancelling in flight calls to the BuyClient
• Added Android Pay support
• Updated OkHttp and Retrofit Libraries
• Consistent use of Long for model ids
• Android Pay support