Blender Git Loki

Move 2.6x versioning code to own file

Summary:
Makes readfile.c more clear and easier to navigate in.
We would need to do such a move when we'll do 2.70 bump
anyway. So better be prepared.

In fact, as soon 2.70 bumo is done i would think we'd
better create versioning_270 file and do version code
there. Otherwise readfile.c will grow up dramatically
again.

Reviewers: brecht, campbellbarton

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D60