Added scripts to configure Bullet Dynamics Engine for building in VC2008.

We have been importing library sources to our source tree directly
for other libraries than Bullet, but Bullet's source is so large
(about 8 megabytes compressed) that I hesitate to include the
whole source into our tree.
After all, we haven't decided whether to use BDE.

Instead of embedding the whole source code, we made a script that
checks out Bullet's Subversion working copy into a subdirectory
and invoke its configuration manager (premake4) to match our
demanding projects.

Also we added a README that describes the method to build it.

The configuration with Win32 and Debug refuses to link for unknown
reason.
Added VC2012 support for Bullet Dynamics Engine.

Actually, Bullet itself does not support VC2012 yet (so do premake4)
but it built well in my case.
Added comments to README.