Bug 859573 - Part 2: API for declaring virtualenv packages; r=aki

This patch provides a formal API for declaring virtualenv packages.
Before, scripts had to override create_virtualenv to install custom
packages. Now, they simply need to make a few API calls at setup time to
register packages and the default implementation of create_virtualenv
will do the right thing.
Bug 859573 - Part 3: Record system resource usage of scripts; r=aki