This updates the main entrypoint for the installer to:


Parse any special command line arguments requested by the caller (for

   unattended installs or specific installation requirements).


Fetch information on the versions of the Review Board packages from

   PyPI.


Start the installation wizard.


With this, the initial version of the Review Board installer is

considered complete.

Completed many interactive and unattended installs, with various

combinations of command line options.
All Docker-based tests pass.