Thread safety using dispatch_barriers

Ported from objective-C, the Swift version is much nicer.
Consistency, fulfill not fulfiller
Revert "Xcode 6.1 GM fixes"

This reverts commit 2b38d736d54f15bef4954cb1a8a99b3de973b3e2.

These modifications were applied too early though half of them are absolutely necessary to avoid edge-case-bad-data crashes :P
Add a playground
Add `thenUnleashZalgo`

Refs #55
Make Swift when work

Closes #72