Allow fast removal of all markers (removeAllMarkers) in api's that support it #167

If an API can support removeAllMarkers in one go, rather than the existing one at a time approach then we should use that, dropping back to the one at a time approach for any APIs that don't have an implementation for it. There is an example for microsoft7 on pull request 166 and an implementation that tries to use removeAllMarkers if it exists and drops back to the current one-by-one approach if none is found.