Eagerly load plugin dependencies

Load `config_builder`, `auto_network`, et. al. as soon as Oscar is required by
Vagrant. The reasoning behind this is that `vagrant plugin install oscar` marks
the Oscar plugin for autloading at the beginning of each Vagrant run. However,
plugin dependencies were only loaded once `Oscar.run` was triggered. This means
that any plugin which needed to hook into Vagrant initialization missed the
chance to perform critical setup.

Fixes GH-28.
Update CHANGELOG and version.rb for 0.4.1