Update 1.5 with Swift 3 support
Update 1.5 with Swift 3 support

- Updated for full Swift 3 compatibility.
- Various optimizations:
- Multiplication is about 2x faster.
- BInt to String conversion is more than 3x faster.
- String to BInt conversion is more than 2x faster.
mkrd/Swift-Big-Integer,mkrd/Swift-Big-Integer,BitBaum/Swift-Big-Integer,mkrd/Swift-Big-Integer