Add support for downloading prebuilt binaries of buck via jitpack at any sha
Add support for downloading prebuilt binaries of buck via jitpack at any sha

Summary:
- Uses Jitpack to build buck pex and serve them on demand. The very first time a version of buck is requested, it is built via [jitpack](https://jitpack.io/). Every subsequent request will just serve the built artifact directly.
- This enables anyone to just fetch `https://jitpack.io/com/github/facebook/buck/<sha>/buck-<sha>.pex` to download a prebuilt buck pex at any sha.
- Once this merges, this functionality will be available for any forks of buck as well, so one can fetch for example `https://jitpack.io/com/github/kageiit/buck/f0655d65907df1262b191339a51aa94a587f9f3c/buck-f0655d65907df1262b191339a51aa94a587f9f3c.pex`

<img width="1034" alt="screen shot 2018-01-18 at 12 36 57 am" src="https://user-images.githubusercontent.com/291148/35073252-6f8a87b0-fbe8-11e7-8f5f-a384818cc04e.png">

- This will allow users of okbuck to not have to rely on checking out the entire buck repo and build it using ant before being able to use buck
Closes https://github.com/facebook/buck/pull/1712

Reviewed By: ttsugriy

Pulled By: ttsugriy

fbshipit-source-id: 15728a4

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