First usable release

• Replace simple-crypt with cryptography for a faster and more common encryption engine.
• Replace the current inheritence implementation with a composition implementation which easily allow to implement additional backends.
• Fix a bug where a stash couldn't be initialized.
• When using the Python API, verify that the value field is always a dict.
• Allow to retrieve a value via stash.get_value(key=key)
• Raise an error if a stash is initialized twice in the same path
• Change the --val flag to --value. --val is just weird.