Update libMesh's AutoPtr class to a modern unique_ptr implementation

(This is a follow-on to #335, it does not need to be merged before 1.0 is tagged.)

This branch defines a new `UniquePtr` type, and gracefully deprecates the old AutoPtr class using a new default configure option, `--disable-unique-ptr`.  When we are eventually ready to break backwards compatibility, we just need to change the default over to `--enable-unique-ptr`.

I've tested this branch with MOOSE, which still uses `AutoPtr`, on OSX, and it works fine.