Update to RxJava2

• Pass all build on local build
• Pass all build on Travis.
• Test sample projects.
  • TodoMVC
  • Counter
  • Todo-with-dagger
  • Todo-with-undo
• Remove RxJava1 dependency from all projects
• Update README.md

Some generated code changed.

When user add observeXXX(BackpressureStrategy strategy), the processor generates these code block.

  @Override
  public Flowable<Counter> observeCounter(BackpressureStrategy strategy) {
     return counterStoreImpl.observe(strategy);
  }