Add docs about new restriction
Handle collections correctly

Fix #132

Passing the collections parameter used to mean that the storage should
append its value to the URL or path. This was a leaky abstraction for
the reasons explained in #132.

The new behavior removes this meaning from this parameter. Vdirsyncer
now maintains a cache of discovered collections.

Also, section names are now validated more strictly, in order to avoid
problems with the status files' naming.