Adapted to native Swift sets + allow optional scalars

[joostlubach]

I've changed three things (of which one is breaking), sorry it's in one commit:

• Attribute members are now created as static struct variables for one-time initialization only (+ I've added direct class var mnemonics: Account.client)
• NSNumber-based attributes now honor the optional flag by adding an optional suffix
• Support for new Swift native Set type.

[kylef]

Thanks for your contribution @yoazt. Would you be able to run make build which will update the test fixtures. Then the tests will pass (and it visually shows the changes in the example).