Package isolation on Mac

We do a few things in this set of changes:
1. We symlink all 'base' and 'recommended' packages into a
'packrat/lib-R' library directory, and
2. Overwrite .Library to point at that directory.

By doing so, on Mac, we now have the packrat private library, and the
symlinked system library, which will now no longer pick up user
libraries that are installed there.

This would then obviate the need for library migration.
Capture only user libraries in 'getLibPaths' as other system libs are implicit
Restore .Library before resetting the library paths
Update NEWS
Use find.package for better resolution of base, recommended packages
normalizePath for Windows
Some is.<platform> utils
Only normalize path on Windows (don't follow symlink)
Some refactoring
Merge pull request #72 from rstudio/feature/symlink-system-libraries

Symlink system libraries on Mac OS X