Swing scheduler

I've added a Swing-specific scheduler to RxJava. However, it probably doesn't belong to `rxjava-core` and therefore, I've created a subfolder named `extensions` with currently one single subproject `rxjava-swing` with currently one single class `SwingScheduler`.

I've tried to create a build for this, too, but I'm not very familiar with Gradle, so somebody should probably check that it's okay like this.

Also, I'm not very happy with the subfolder name (`extensions`). Maybe somebody has a better idea for a more specific name?