Add support for SUGAR_PROFILE_NAME variable

If this environment variable is set, we skip the intro UI
and just setup the profile using that name.

This is going to be useful to run automated tests of the
html activities.
Add support for SUGAR_RUN_TEST variable

When set the sugar shell runs a subprocess with
the command line specified by the variable, watches
it and exits with the same status code.

This will make it easy to run tests inside the shell
environment, html activity tests in particular.