Add default proxy settings from System proerties

Aether apparently doesn't use the java.net.* APIs for
TCP connections so it doesn't notice when a user sets
-Dhttp.Proxy*. To fix it is painful, and leads me to
suspect that actually we might want to parse a
settings.xml at some point (however unpalatable that
is).

For now I have added a Proxy to all RemoteRepository
instances that we create in the CLI if the user
has set -Dhttp.proxyHost (and/or -Dhttps.proxyHost for
a secure repository).

TODO: authentication. Is there a standard way to specify
that globally via system properties.

TODO: maybe use per-repository settings if provided
(e.g. in settings.xml).