Default to path='/' in base_url
Default to path='/' in base_url

The `base_url` function now assumes '/' as the default path. Instead of
setting '/' as default parameter, it is only added if there's no domain
part, as adhocracy defaults to URLs without trailing slash.

This fixes #146.

DanielNeugebauer/adhocracy,alkadis/vcv,liqd/adhocracy,phihag/adhocracy,alkadis/vcv,phihag/adhocracy,alkadis/vcv,SysTheron/adhocracy,alkadis/vcv,liqd/adhocracy,liqd/adhocracy,liqd/adhocracy,SysTheron/adhocracy,alkadis/vcv,DanielNeugebauer/adhocracy,phihag/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,SysTheron/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy