Update find_package(Simbody) mechanism.

Simbody uses a new mechanism for client projects to find Simbody. Also,
Simbody's installation was reorganized on non-WIN32 platforms. To address both
of these, I previously changed OpenSim's find_package(Simbody ...) mechansim to
operate differently depending on the platform (WIN32 or otherwise). In this
commit, I remove the platform-dependent behavior, and include an error message
for if Simbody is not found.
Fix finding simbody for re-configuring.

Previously, the mechanism for finding simbody was not working well if the
user wanted to override the Simbody installation that CMake first found.

Now, when finding Simbody, we ignore any Simbody that we have previously
found when the user ran CMake's "Configure".

If the user provides a SimTK_INSTALL_DIR, we only look there for Simbody.
Otherwise, we let CMake search for it.
Fix comment related to find_package(Simbody).
SimTK_INSTALL_DIR->SIMBODY_HOME.
[travis] Use SIMBODY_HOME instead of CMAKE_PREFIX_PATH.
Merge pull request #98 from opensim-org/improve-find-simbody

Improve find simbody