add SAXConverter for XML deserialization

Per issue #193, this adds support for SAX XML deserialization, supported equally across java and android.  STAX is also supported on android, but I personally use SAX more often.

This PR introduces `SAXConverter.Deserializer`, adding the ability to read the result of `ContentHandler`.  A Deserializer is created per type, and this is expected to vary with implementations.  Accordingly, `SAXConverter.newDeserializer(Type)` is marked abstract.  

An example implementation could look look like this, though those using Dagger will surely optimize it.
```
  static class STSConverter extends SAXConverter {
    @Override
    protected Deserializer newDeserializer(Type type) {
      if (type == SessionCredentials.class)
        return new SessionCredentialsHandler();
      else if (type == AWSError.class)
        return new AWSErrorHandler();
      throw new UnsupportedOperationException("type not yet supported: " + type);
    }

    @Override
    public TypedOutput toBody(Object object) {
      // serializing to XML can be done many ways including
      // something simple as StringBuilder or String.format
      throw new UnsupportedOperationException();
    }
  }
```