Moving unique_ptr.hpp to its own contrib directory.

Ultimately I decided to install unique_ptr.hpp alongside the libmesh
headers, similarly to nanoflann.hpp, since trying to include it with
our Boost seemed like it would be difficult to make work with external
Boost installations.

Creating a new contrib directory was a simple 5 step process:

* Create contrib/unique_ptr/Makefile.am from scratch, copying from nanoflann's.
* Adding an AC_CONFIG_FILES line in libmesh_optional_packages.m4 for the new
  Makefile.am to get automake to see it.
* Update the include/libmesh/rebuild_makefile.sh script by hand to include the
  unique_ptr.hpp header.
* Updating include/libmeshMakefile.am file by running the script.
* Running bootstrap.
Ran bootstrap