More permissive hostname regexp

The server field should be allowed to contain uppercase letters
and numbers.  We should convert it to lowercase when storing the
entry, for consistency with the rest of the implementation.
Include malformed line in ValueError args

Actually tell the caller which entry we're unhappy about.
Provide a short_name attribute in class Meeting

Provide a convenience attribute ('short_name') that is the last
component of the path.  c.f. 'long_name'
Cleanup semantics of touch()

We should only bump the highest-read transaction if we've read a
newer one.  We always bump the timestamp.