changes for Swift beta 4 compatibility

This was mostly just marking (nearly) everything as public. The other
changes were the switch to using stride(), a NSRange subscript for
String, and a few other minor syntax changes.
added string justification methods

Added str.ljust, str.rjust, and str.center.
Merge pull request #8 from natecook1000/master

changes for Swift beta 4 compatibility
Now using [T : Void] instead of [T : Bool].
Formatting.
Added set-tests.
Document work-around.