added warning if IMU time lags behind ROS time
use ros::Time::now() if time lag exceeds threshold
add some hints to error message

I just spent 30 minutes trying to figure out why the IMU works on one
computer and doesn't on another one. Felt a little foolish when I found
out that the udev rules weren't installed; maybe providing some more
info in the error message helps others.
Merge pull request #2 from uos/update_errmsg

add some hints to error message
Merge pull request #1 from uos/fix_imu_time_lag

fix IMU time lag
Merge pull request #3 from ccny-ros-pkg/master

Merge branch 'master' into fuerte