Publish OSX Binary

[rbuckland]

The OSX Binary release is missing from

https://github.com/rust-lang-nursery/mdBook/releases

It was removed as part of OSX Build failures #549

But erroneously the README.md suggests that Linux and OSX are built (Build status at the top)

[mattico]

Yes, I have no way to build these locally but I do intend to get CI releases working again.

[badboy]

I think it's still best and easiest to rely on trust for easy compilation on the required platforms (probably x86_64 and i686 linux as well as x86_64 macos)

@mattico:
Would you be ok with bringing back trust again to enable linux and mac builds? I'd be happy to send a PR.

[ehuss]

Macos builds are working again in the latest release: https://github.com/rust-lang-nursery/mdBook/releases/tag/v0.3.0