First set of changes.
Simplified use of SimpleCrypt
* Changed allocation of SimpleCrypt from heap to stack. This
  simplifies its use a little as we don't need to keep track
  of construction and deletion.
* Removed manual deletion of QNAM as it's parent is already set
  and Qt wuill take of deleting children when parent is deleted.
* Added a few string contants which are being used repeatedly.
* Added a helper function for generating the key to be used for
  encrypting/decrypting stored tokens.
Second set of changes
Minor code cleanup

* Simplified concatenation of lists in getRequestBase().

* In O1::onTokenRequestFinished(), added extra check to if
  "oauth_callback_confirmed" is set to true. This is required
  by RFC 5849 (section 2.1) and is also menioned in the
  twitter API docs here:
https://dev.twitter.com/docs/auth/implementing-sign-twitter

* Removed the use of string constants done in previous commit
  as I feel, I can do ALL such conversions in a separate
  commit which will be easier to review.