Release of version 0.99.2

The long-awaited release 0.99.2 (sorry, still not 1.0) contains a bunch of new elements: input, checkbox, button, reflexangle, nonreflexangle, majorsector, minorsector. Groups got an overhaul, it is now possible to rotate and scale groups. See the JSXGraph API reference for examples. Further, curve plotting got a major speed up, axis labeling has been improved, and curves can now be made draggable. As always, many bugs were fixed. See the CHANGELOG for a more detailed list of changes. This release is based on the contributions of S. Blanco, M. Bourne, benbro, A. Elkins, P. Platt. We are very happy that the list of external contributors for JSXGraph is increasing.