Fix scope of _tileFunction
Update leaflet version
Supporting promises

- Supporting promises instead of just jQuery deferreds.
- Added demo for promises.
Support maxNativeZoom

- See issue
https://github.com/ismyrnow/Leaflet.functionaltilelayer/issues/3
- and issue https://github.com/Leaflet/Leaflet/pull/1802
Fire tileloadstart event

- Also convert some double quotes to single quotes.