Initial Swift 3 compatibility

[jakeheis]

These changes are the minimum required to make FileKit work with Swift 3. This means that the following were not changed, but perhaps should be addressed in a future pull request before a final 4.0 release:

• Access control could be reconsidered - Xcode automatically converted public to the now equivalent open and similarly private to fileprivate, though perhaps the new public and private would be more appropriate in some situations
• Methods names were converted by Xcode to maintain the same API, not to have more Swifty names. For example, func symlinkFileToPath(path: Path) was changed to func symlinkFileToPath(_ path: Path), while perhaps its eventual name should be func symlinkFile(to path: Path)

While these could be addressed in this pull request, I believe they would be better served by a future one.

The only consideration left in this pull request is Path's conformance to Indexable. For a reason I could not deduce, Swift 3 throws errors when Path conforms to both Indexable and Sequence, so in this pull request I removed conformance to Indexable in order to fix the errors. The Indexable conformance is not actually utilized anywhere, so I think it would be best to just remove it, but I'm happy to do whatever you think is best in this situation.

[nvzqz]

This fails to build on iOS due to missing DispatchFileSystemWatcherDelegate. Please review your changes.

[jakeheis]

Whoops 🙈 sorry about that, shouldn't have done the final testing only on the OSX target. Should be good now