Allow config files without sections

[Giampaolo Rodola]

...because most of the times this is what we want.
The way I imagine this.
With no section:

@register()
class conf:
    foo = 1

With sections:

@register(section='base')
class conf:
    foo = 1

Since this is still super-beta software as a first step it makes sense to remove the current behavior supporting sections.