Properly unsubscribe Layer events 

I've just bumped into this while browsing the source code. It's not tested in any way and I didn't look for similar issues in other files neither. But I hope it helps anyway.
Merge pull request #3084 from miroslavraska/patch-1

Properly unsubscribe Layer events
try out the new fast Travis build thingy
Revisions to CONTRIBUTING.md
Merge pull request #3091 from markjszy/docupdates

Revisions to CONTRIBUTING.md
Hide single base layers in the Layers control.
Disabled by default so it doesn't break existing apps.
Merge pull request #3093 from cebence/no-single-base-layer-in-layers-control

Hide single base layer in the Layers control