Added emacs tilde files to .gitignore
Provide for additional options and extensions to Encrypted Fields

- Use per-model database keys if specified
- Optionally prefix encrypted data before storing in the DB.
- Provide a 'decrypt only' mode where the model will only decrypt
  data, but not encrypt ever. That duty is the responsibility of the app.
- Optionally enforce max_length
- Provide for a hook to user-specified keyczar derived classes/wrappers.