Fix dbname parameter for isql and create_database

Valid sintax:
dbname
host:dbname           -- host & dbname
host/port:dbname      -- host & port & dbname
localhost/port:dbname -- port & dbname
A better name for the format_uri method? Or even a better implementation...
Rename format_uri to connection_string and have it handle host without port.

And add some tests to make sure it works properly. It also throws an exception
if there is no database name.