Standardize install paths and cmake finder

I've made some changes which should approach Simbody to the standard guidelines described by Debian policy and AFAIK they should be quite cross-platforms improvements, although testing in Windows/MacOsX would be nice. Detailed list:

1. Use GNUInstallDirs, which should provide canonical paths aware of the platform.
2. Change all the hardcoded paths to use the ones provided by GNUInstallDircs (CMAKE_INSTALL_DOCDIR, CMAKE_INSTALL_LIBDIR, etc).
3. Adapt and change name to cmake finder, generate it from a .in file to be aware of CMAKE_INSTALL_LIBDIR. 

I will move the debian/ directory out of the source, but I have leaved here during the pull request so the great @thomas-moulard can review the debian metadata changes and maybe provide some feedback.