feat: port to RxJava 2.X

[kaushikgopal]

See #82 for thought process behind how this is being done.

RxJava 2.0 has hit stable, so it is time to port these. Here's what's different in 2.0. For a lazy quick helpful summary, you can check the fragmented episode with Jake.

Both 1.x and 2.x will coexist temporarily until we're done with the migration. After that all references to 1.x can be removed.

• add 2.x dependencies
• port example Background work & concurrency (using Schedulers)
• port example Accumulate calls (using buffer)
• port example Instant/Auto searching text listeners (using Subjects & debounce)
• port example Networking with Retrofit & RxJava (using zip, flatmap)
• port example Networking with Retrofit & RxJava (side by side with AsyncTask)
• port example Two-way data binding for TextViews (using PublishSubject)
• port example Simple and Advanced polling (using interval and repeatWhen)
• port example Simple and Advanced exponential backoff (using delay and retryWhen)
• port example Form validation (using combineLatest)
• port example Pseudo caching : retrieve data first from a cache, then a network call (using concat, concatEager, merge or publish)
• port example Pseudo caching : old merge e.g. with result age strategy
• port example Simple timing demos (using timer, interval or delay)
• port example RxBus : event bus using RxJava (using RxRelay (never terminating Subjects) and debouncedBuffer)
• port example Persist data on Activity rotations (using Subjects and retained Fragments)
• port example Persist data on Activity rotations (using Subjects and retained Fragments) old variant
• port example Networking with Volley
• port example Pagination with Rx (using Subjects)
• port example Timeout
• add Timeout example to README
• port PlaygroundFragment
• remove 1.x dependencies
• update README to point to this PR as a migration guide

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thanks @marukami + @marcinkunert for lighting the fire on this

[kaushikgopal]

as @marcinkunert pointed out RxBindings (which we use in a couple of examples) hasn't been ported yet.

Option 1: we use karnok's excellent Interop lib
Option 2: we remove RxBindings and use Subjects or the like to achieve the same functionality (which we sort of do already in the double binding example)

i'm leaning towards Option 1 just because i think that's the more realistic situation for us in our real world apps. Option 2 would be cleaner but it's crazy to have to re-write or emulate functionality already being provided through other libs.

k let's go with Option 1 for now 👍

[marukami]

+1 for Option 1
It's not just RxBindings that's still 1.x; Realm is also still 1.x. The bindings examples should be useable instances in both contexts.

[kaushikgopal]

yup. was actually just looking at Realm's support the other day. Given Realm's open source, once we're done here, should help with the migration there :P.

above being said, since we don't use Realm in any of the examples i would have been inclined to not worry about it. but you're right the more i think about it, there are way too many libs that are useful and haven't been ported to 2.x yet. makes sense to also demonstrate how the interop would work