Remove function bodies from externs
Add ol.source.Cluster

Initial code by @kenny806
Update link to closure-library issue
Merge pull request #2464 from fredj/closure-library-issue-link

Update link to closure-library issue
Merge pull request #2463 from gingerik/externs-function-bodies

Remove function bodies from externs
Merge pull request #2458 from openlayers/cluster

Add clustering functionality
Remove unused ol.CoordinateArray typedef
Add workaround for mousewheel event name

'DOMMouseScroll' for Gecko and 'mousewheel' otherwise.

See https://github.com/google/closure-library/pull/308
Merge pull request #2466 from fredj/unused-typedef

Remove unused ol.CoordinateArray typedef
Merge pull request #2467 from fredj/2460

Add workaround for mousewheel event name