[mylocation] fix missing lon; add locate & find-marker buttons

[thinkpoop]

Link to app

Fix for Longitude ending up undefined

The map (leaflet.js) uses "lng" for longitude, while the MyLocation.json uses "lon". When re-loading the lat/lon from JSON, the original code wasn't mapping "lon" to "lng".

To recreate using BangleJS.com/apps version:

1. Open the interface, add a marker, and click save.
2. Re-open the interface and click save without clicking on the map.
3. MyLocation.json will have an undefined longitude; quick test is just re-open the interface again and it'll give an error in the console about it.

Adding Locate and Find Marker buttons

Adding a "Locate" button to center on your current location and a "Find" button to center on your marker's location.

What the Locate button looks like before setting a marker

What both buttons look like after setting a marker

[bobrippling]

Nice spot - one small question

[bobrippling]

Thanks!