Add ol.source.Source#dispatchChangeEvent
Propagate change events from sources via layers
Merge pull request #918 from twpayne/source-change-events

Source change events
Add getParams and updateParams functions in singleimagewmssource
Add getParams and updateParams functions in tiledwmssource
Add getContext and updateContext functions in wmtssource
Fix functions description and parameter name
Fix variable name
Replace context and kvpParams variable by th property params_
Allow tile cache key calculation to be overridden
Add custom getKeyXYZ function in wmtssource and call dispatchChangeEvent() when updateparams is used