Updates for Swift 1.2

[vittoriom]

I ran Xcode migration tool. After that, I had to apply some manual changes:

• I removed the assigned value to the isConcurrent DispatchQueue constant. It was confusing for me to see it re-assigned in the initializers
• I also assigned the gcd.serial() value to the queue constant inline with its declaration in DispatchTimer
• I had to remove the sync and async implementations that accept an @autoclosure callback in the DispatchQueue class because of how autoclosure works now in Swift 1.2 (in particular, @autoclosure implies @noescape, that certainly doesn't work for the async method, but seems to not work for the sync either, due to some issues with the dispatch_sync (that obviously doesn't mark its parameter with @noescape or @autoclosure).
• Of course I had to update the tests code to explicitly use a closure where @autoclosure was used before.

Let me know if you need something else

16 tests are passing.

[aleclarson]

Wow, thanks for your great work! 😄

I'm just going to auto-merge this and hope it all works right. Don't have any time to spend on this.

Thanks again!

[vittoriom]

No problem! We are using your library and I thought it would have been nice to give something back :) Everything should work, assuming the tests are correctly testing :)
Worst case, next week we'll try the 1.2 version out and submit some other PR if something is not working :P

[aleclarson]

👍