Revise hashing at NSData and NSString extensions

[unpluggedk]

• Remove hashing from NSString extensions
• Add convenience converting method for NSString -> NSData
• Modify NSData extensions to handle hashing, modulized.
• Add stringify methods to NSData.

example calls

NSString *sha256Hex = @"string".msidData.msidSHA256.hexString;
NSString *sha256Base64 =  @"string".msidData.msidSHA256.base64EncodedString;
NSString *sha256Base64UrlEncoded = @"string".msidData.msidSHA256.base64EncodedString.mdmsidBase64UrlEncode;

[unpluggedk]

Addresses #147

[oldalton]

Looking good, but would you mind adding some tests to this. Based on your changes it looks like we didn't have any tests for it before :) e.g. a couple of tests for hex string

[unpluggedk]

#221 should address this