When running an application, --local can be used to collect the
application's dependencies in a local directory. Prior to
AetherGrapeEngine being introduced, using --local would result in the
dependencies being written to ./grapes. When AetherGrapeEngine was
introduced --local no longer had any effect.

This commit updates AetherGrapeEngine so that it honours --local,
writing its dependencies to ./repository. When --local is not specified
dependencies are written to ~/.m2/repository (the standard location
for the local Maven cache). As part of this change TestCommand has
been refactored so that it lazily initialises its GroovyCompiler. This
ensures that RunCommand has a chance to set the system property that
backs --local before AetherGrapeEngine is initialised and accesses the
property.

Fixes #99