Update README.md to use correct API

[ghost]

The Complex use case example incorrectly uses the API.

Appears to use outdated X509PublicKey() rather than publicKeyDataX509()

Proposed Complex use case:

let localHeimdall = Heimdall(tagPrefix: "com.example")
if let heimdall = localHeimdall, publicKeyData = heimdall.publicKeyDataX509() {

    var publicKeyString = publicKeyData.base64EncodedStringWithOptions(.allZeros)

    // If you want to make this string URL safe,
    // you have to remember to do the reverse on the other side later
    publicKeyString = publicKeyString.stringByReplacingOccurrencesOfString("/", withString: "_")
    publicKeyString = publicKeyString.stringByReplacingOccurrencesOfString("+", withString: "-")

    println(publicKeyString) // Something along the lines of "MIGfMA0GCSqGSIb3DQEBAQUAA..."

    // Data transmission of public key to the other party
}