• Added new abstract class, URI::_odbc, for URIs that use ODBC in
  dbi_dsn.
  
• The dbi_dsn method of URI::mssql now returns an ODBC DSN instead of
  DBD::Sybase, since the latter would require Sybase to build.
  
• URI::sqlserver now inherits from URI::mssql rather than the other way
  around.
  
• Fixed a bug where a URI with three slashes but no authority part after
  the first two would incorrectly think the databse name should be an
  absolute path. That is, in "db:sqlite:///foo.db", the dbname value
  is "foo.db", not "/foo.db". Thanks to Dan Book for the report
  (issue #8).