add automatic tf buffer cleaning on bag loop for python

This logic was already implemented for c++
but not for the python module.
ignore tiny backjumps in loop detection

For some reason rospy's now() can produce negative
increments by 0.01-0.05 seconds on our hardware.

This needs to be investigated, but this is a temporary fix
Merge pull request #37 from v4hn/python-tf-loop-clean-buffers-fuerte

add automatic tf buffer cleaning on bag loop for python