Time synchronize multiple uBlox messages to get a single callback

Port of message_filters::ExactTime message synchronization policy to use integer time of week in milliseconds (iTOW) instead of ros::Time in a header.

The message_filters::ApproximateTime message synchronization policy is not relevent because messages generated by a ublox sensor for a single update contain identical iTOW time stamps.

See src/example.cpp for example usage.

Launch the example with roslaunch ublox_msg_filters example.launch. The example is not installed, so this must be in a source workspace.