Improved Simbody's version-checking mechanism.

This requires CMake 2.8.6 (only 2.8 was required previously). We use a script
to automatically generate a SimbodyConfigVersion.cmake file. Previously, I
wrote one myself, but it was likely to be buggy.

This new script enforces the behavior that changes to the major version number
are backwards-incompatible, but changes to the minor or patch versions are
backwards-compatible.

I tested this on Linux, with OpenSim as a dependent project.
Fix the module I tried importing.

I want to use WRITE_BASIC_CONFIG_VERSION_FILE, which in CMake 2.8.6 is only
in the module WriteBasicConfigVersionFile.
Update README with new required CMake version.
Fix name of macro for config version file.

I was using the name of the macro in a newer version of CMake.
Merge pull request #161 from chrisdembia/version-config

Improved Simbody's version-checking mechanism.