Changed reference frame from base to laser to account for upside down and/or back facing laserscanners

Since we were having some problems with a rear facing, upside-down mounted laserscanner, we have rewritten the wrapper to use the laser-frame as base for gmapping and leave the transformation of scan-positions entirely to the wrapper class. This should make slam_gmapping much more robust to non-standard mounting positions of laserscanners.

Here is [a bag with two launch files](http://users.informatik.haw-hamburg.de/~aba879/ros/bag_scitos_2013-04-08_scan_odom_tf.tar.gz), that demonstrate the problem.
The front-scanner works fine, but the rear facing one produces a messed up map in standard gmapping.


A nice side-effect of these changes is, that gmapping can now handle panning scanners, as the base->laser transform is now dynamically included in the gmapping pose.

The .gitignore file commit slipped in but should be useful nevertheless.

A problem with non symmetrical scans (e.g. min_angle = -1, max_angle = 2) still exists and may be fixed in a future version. (This is not new, but also a problem of the current implementation.)