Make `isValidIP()` backwards compatible with deprecated `is_valid_ip()`.

The old `bridgedb.Bridges.is_valid_ip()` function expected a single
string parameter, and would return a boolean. The new
`bridgedb.parse.addr.isValidIP()` function expected an
`ipaddr.IPAddress` as a parameter.

 * CHANGE `bridgedb.parse.addr.isValidIP()` to convert any strings which
   it receives as arguments into `ipaddr.IPAddress`s, which makes it
   backwards compatible with the (deprecated)
   `bridgedb.Bridges.is_valid_ip()` function.