Linux support.

[ratranqu]

See: https://github.com/krzyzanowskim/CryptoSwift/blob/master/Sources/CryptoSwift/Multiplatform.swift
And: http://blog.krzyzanowskim.com/2015/12/04/swift-package-manager-and-linux-compatible/

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #5 into master will not change coverage

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[lorentey]

Awesome!

[lorentey]

I'm a little worried about how this shim seeds the Mersenne twister; I assume it won't just produce the same series after every startup, would it? arc4random plugs into the kernel random pool, so it provides relatively high quality entropy.

Also, it would be better to fill the buffer by generating four bytes at once (or eight if mt19937_64 is also available); there is no need to go byte by byte.