update readme badges

Make Hash.Function digestLength public

support Swift 3

Improvements

Fix for rand() error in Swift 3 (#20)

When playing with a package that was related to this package, I found that Swift 3 gave the following error:

    /Users/mhughes/Projects/Kitura/Packages/OpenSSL-0.8.4/Sources/OpenSSL/Certificate.swift:86:16: error: 'rand()' is unavailable in Swift: Use arc4random instead.
                    let serial = rand()
                                 ^~~~
    Darwin.rand:2:13: note: 'rand()' has been explicitly marked unavailable here
    public func rand() -> Int32
                ^
    <unknown>:0: error: build had 1 command failures

This patch makes that small change.

Also includes support for Linux and Apple platforms, but doesn't support any/all other platforms (Windows, BSD, etc.) at this point.

Fixing rand issue

Merge pull request #19 from pvzig/linux-compilation-patch

Linux compilation patch

add sha256

add message digest

Additional warning fixes (#18)