Added support for symbol-like notes

• ➕ Notes are first class variables, string wrapping not required. Examples: a (A3), bs (B#3), db (Db3), fx5 (F##5, enharmonic to G5), ebb (Ebb3, enharmonic to D3)
• ⬆️ Minimum node version now 6.0.0 (for Proxy support, above)
• 🔼 Upgraded to [email protected]
• 🐞 No crash when no MIDI controller available