adding freebsd install script strategy

It works as follows:
 - Look for a phantomjs binary in PATH that with the correct version
   number
 - Find out where that binary is located
 - Copy the binary to lib/phantom
 - Make the copied binary executable (755 perms)

This way the script does not concern itself with how phantomjs was
installed (either from source or ports), it just copies over whichever
one is in your PATH variable.