Support Buck build

[nguyenhuy]

First attempt to support Buck 🐇 🏎 💨

How to use:

# Run this once on the root directory to resolve dependencies (i.e PINRemoteImage, PINCache, OCMock, etc)
# and populate their BUCK files (more on this later)
pod install

buck build lib
# Same as above because 'lib' is an alias for //:AsyncDisplayKit
buck build //:AsyncDisplayKit
buck build //:AsyncDisplayKit-Core

buck test

Dependency resolution:

• Currently I build dependencies from source files downloaded by CocoaPods.
• There is a post_install hook added to the Podfile in root dir. The hook copies all BUCK templates needed to build the pods. Our main BUCK file (in the root dir) has hardcoded paths that point to these dependent BUCK targets. That means pod install needs to be run at least once.
• Buck can already build static and shared prebuilt frameworks (prebuilt_apple_framework). So we can rely on Carthage for dependency resolution if needed to. I used it to build FLAnimatedImage for a while, but decided to abandon this approach because prebuilt_apple_framework is still under development and poorly documented.
• A BUCK template has just enough information to build source files of a downloaded Pod. That means it either is an exact copy of the BUCK file from original repo (in case of PINCache), or is a stripped down version of the original BUCK file (in case of PINRemoteImage if we use pod 'PINRemoteImage/iOS', ' 3.0.0-beta.7' and pod 'PINRemoteImage/PINCache' in Podfile), or only lives in our repo (in case of FBSnapshotTestCase, OCMock, etc).

Remaining/followed-up tasks:

• BUCK files for all examples
• HTTP cache for build artifacts 🏎 💨
• Benchmarks
• CI integration

This PR depends on #2844.

Let me know if you have any questions or suggestions!

[nguyenhuy]

Here are screenshots of 2 consecutive test passes (bct is my alias for buck clean && rm -rf buck-out && buck test). The first clean build and test took 52.8 seconds in total: 22.4s for building (without HTTP cache) and 30.2s for testing. The second pass took 700ms because everything is cached, including test results!

[Adlai-Holler]

I don't know much about Buck but this looks beautiful! Faster builds and more smarter build system = 👍I support landing this whenever you feel it appropriate.

I would be good if possible to separate out the Buck support from the ASVideoNode delegate change, no?

[nguyenhuy]

@Adlai-Holler: Thanks for the review. The ASVideoNode delegate change was merged in #2844. I've rebased this PR to remove that commit. Let's wait for another day so others have a chance to raise any questions/concerns. Also, I can't land this myself (no GitHub permission) :)

[Adlai-Holler]

👍

[Adlai-Holler]

Here we go!

[nguyenhuy]

Very excited. Thanks!