* dtls/openssl.py:
- Added class _EllipticCurve() for easy handling of the builtin curves
- Added wrapper get_elliptic_curves() - which uses _EllipticCurve()
- Added EC_get_builtin_curves(), EC_KEY_new_by_curve_name() and EC_KEY_free()
- Added OBJ_nid2sn() for translating numeric ids to names
* dtls/util.py: Added _EC_KEY() derived from _Rsrc() with own free/del method