make datasource parser handle empty properties
  * datasources like "erbi:driver" and "erbi:driver:" should work
  * added unit tests for these cases
fixed bug in erbi_statement
  * erbi_statement:get_rows was making bad continue_fetch call, and not
    accumulating rows properly when fetch all returned more than one
    block of rows
  * added tests which set the flags to have dummy driver simulate this
    (including making versions of the funcs which get connections to allow
     passing params)
  * fix dummy driver's simulation of multi-fetch