Fixed a bug with automatic $PATH generation.

Fixed an issue with FakeInstall that would generate incorrect values for $PATH when the script itself was called with a relative path (e.g. ./FakeInstall instead of just FakeInstall).  Also factored $PATH assertion into its own function in Common.sh.
Added UpdateD.

UpdateD is a new script to install/update DMD.  This commit also includes an accompanying addition to the readme.