Added support for SHA256 hashed passcodes

[mattcroxson]

The hashedCode property can be used to signify that the passcode property will either return or be set as an SHA256 hashed string. Also included is the MIHashing class so an SHA256 hash string can easily be generated using [MIHashing hashForString:].

The CommonCrypto framework is required in addition to QuartzCore.

[dhennessy]

Where does the MIHashing code come from? In particular, is it open source under the same license?

[mattcroxson]

Hi,

It's a class I put together inspired by various posts on Stack Overflow. Licence wise I consider it to be open source.

[dhennessy]

Gotcha - thanks. Give me a day or so to review the changes.