Added support for Swift 4.

This release minimally requires use of the swift-3.1.1-RELEASE toolchain or the swift-4.0.0-RELEASE toolchain which is recommended.

• Compatible with Xcode 9.0 (9A235) General Release or higher using one of the above toolchains.
• Removed old restriction on Digest by now allowing the Data extension to work on Linux as well as macOS.
• Renamed Updateable protocol to Updatable.
• Added new Data based update function to Updatable.
• Added support for SHA1 algorithm in both Digest and HMAC.
• For every function that took a NSData, there is now an equivalent function that takes a Data object.
• Added tests for new Data based functions.
• Allow use on iOS, tvOS and watchOS as well as macOS.
• Removed the inout modifier from the update(data: Data) function in Updatable extension. It was unnecessary. See issue #7 for more details.
• Added CI (PR#6).
• Removed RSA stub. This will be replaced by a forthcoming RSA-centric cross-platform package in the near future.
• Merged PR #20 to fix a memory leak on Linux.
• Added iOS target to supplied Xcode project.
• Merged PR #23 to improve documentation formatting.
• Support of Xcode 9 Beta.
• Support for Swift 4.
• Detect an invalid initialization vector size passed in to the StreamCryptor initializers. Issue #26.
• Fixed problem with passing the size of an IV passed as a String. Need to account for all the bytes not just the character length which could be different.
• Added additional tests to cover issue #26.
• Added CocoaPod support (not quite functional yet).
• Remove build on Xcode 8.3.
• CI updates.
• Completed audit of fatalErrors as per issue #30 . Next step will be the code changes. No code changes with this release, just added comments where fatalError() calls will be removed in a future release and replaced by exceptions and/or Status changes.