Auto-Fetch parity on first run

[amaury1093]

The behavior is:

• if an instance of parity is running, run UI normally
• if not, then fetch latest version of parity (as per vanity-service) and put it in the Application data folder (so not in PATH). A progress bar is shown during this step. The UI will then run parity in the bg once installed.
• all --flags passed to Parity UI will be passed down to parity

Closes #57
Closes #65