Deprecate b.Bridges.is_valid_ip() in favour of b.parse.addr.isValidIP().

 * MOVE `bridgedb.Bridges.is_valid_ip()` function to
   `bridgedb.test.deprecated` module, and mark it as deprecated in
   bridgedb-0.2.3. `bridgedb.parse.addr.isValidIP()` should be used
   instead.

 * ADD a line to the integration tests monkey patcher in
   `bridgedb.test.test_Tests.monkeyPatchTests()` to patch
   `is_valid_ip()` back in for testing as necessary.
Deprecate b.Bridges.BridgeHolder in favour of b.hashring.Hashring.

 * MOVE `bridgedb.Bridges.BridgeHolder` class to the
   `bridgedb.test.deprecated` module, for patching into integration test
   runs as necessary.

 * CHANGE inheritance for several other classes in `bridgedb.Bridges` to
   inherit from `bridgedb.hashring.Hashring` instead of `BridgeHolder`
   class:
     - bridgedb.Bridges.FixedBridgeSplitter
     - bridgedb.Bridges.FilteredBridgeSplitter
     - bridgedb.Bridges.UnallocatedHolder
     - bridgedb.Bridges.BridgeSplitter