Make "." be the CLI's default classpath

Previously, the default classpath was empty. Now, in the absence of the
user providing a classpath via the -cp option, the default classpath
will be ".". If the user does specify a classpath, the classpath that's
used will be exactly what they have specified, i.e. "." will no longer
be on the classpath unless specified by the user.

The app sample integration test has been updated to verify that "." is
only the classpath by default.

Fixes #115