The writeFeatures method should always return a string

This PR fixes  #2102

All writeFeatures, writeFeature and writeGeometry methods now return a string. The respective methods such as writeFeaturesObject (JSON) and writeFeaturesNode (XML) have been marked api but not stable as yet.
Okay I added a testcase to ol.format.KML to use writeFeatures and it passes in IE9, there are other failures in IE9 though but unrelated to this PR (see #3012)