Added variadic rendition for `executeUpdate` and `executeQuery` for Swift as "extra"

Pursuant to https://github.com/ccgus/fmdb/issues/291, I have added the `FMDatabaseVariadic.swift` extension to `FMDatabase`, which enables the variadic renditions of `executeUpdate` and `executeQuery`.

BTW, I added this Swift extension in the "extra" directory, as I didn't want to introduce errors for Objective-C users. I also didn't touch the CocoaPods stuff, either, but presume we'd want to do something in that regard for the poddists out there.

I also moved the two `FMDatabase+InMemoryOnDisk.h` and `.m` into a `InMemoryOnDiskIO` subdirectory, which seemed more logical to me.