Create field bases and instances during install, not enable

Follow-up from #1572578-20: Rethink the way features are enabled / disabled.

Another problem that we've encountered with Features is that you cannot create nodes during an install.

Features only creates the field bases / field instances on hook_enable(), which occurs after hook_install(). So basically if you have a feature that adds some fields to (for instance) Page, you cannot create a Page during hook_install() of, say, a profile.