refactor core into several modules (fixes)

This is a fixup to 6eeb79b18bd2b4dfab3969d9d6328cf3de7de301.
timestamp transforms slighly into the future

This is necessary for the frustum drawn by
examples/ros/draw_rviz_frustum.py to be visible in rviz.

This reverts 454eaf9d57f3447c20bb918b2eed261b8d3ed00c.
move ROS camera publisher into main pymvg library
py3: fix string comparison check

This fixes the rather broken 8ca65a2c669423b596259eb7163c748f39e7cbcd
testing: add test for camera equality
bugfix: actually test equality of cameras
travis CI: show test results on master branch
testing: add synthetic camera file
bugfix: use correct method call
examples: make ROS example just like plotting example
testing: simplify ROS roundtrip test
CameraModel: rename constructor to be more private
bugfix: fix CameraModel's notion of ROS transform