Migration to RxJava2

[pwittchen]

Things to do:

• [x] migrate code
• [x] migrate documentation
• [x] update Gradle config and settings
• [x] add appropriate info about Rx1 and Rx2 to both git branches
• [x] update gh-pages for both branches
• [x] remove master branch

Remember about keeping backward compatibility with RxJava1.x.

[pwittchen]

Code was migrated in https://github.com/pwittchen/prefser/commit/0c5d5adbf6f335800d80f637596a7e49c2a1e2f2.