Merge pull request #33 from hsu/simbody-3.3

Prevent NaN generation when gravity is (0, 0, 0) and a previously-excluded body has gravity re-enabled. This needs to be merged into the master branch also.
fix #38: use CMAKE_INSTALL_PREFIX

SimTK_INSTALL_PREFIX was previously used for specifying the
install prefix and CMAKE_INSTALL_PREFIX. This commit changes
to use CMAKE_INSTALL_PREFIX instead.
Issue #38: fix use of SimTK_INSTALL_PREFIX

Fix the use of SimTK_INSTALL_PREFIX for backward compatibility.
Minor modifications to allow compilation with Visual Studio 2013
(a.k.a. VS 12). Some obscure missing std:: functions were finally
added, and std::max and std::min require <algorithm> to be
included now.
- Modify CMakeLists to avoid warnings in VS 2013 64 bit builds (/arch:SSE2 unrecognized). SSE2 is the default for that compiler.
- Fixed caching problem that prevented BUILD_INST_SET changes from taking effect. This required removing the compiler command line variables from the cache; those are no longer changeable from CMake.
Update version number to 3.3.1.