Merge pull request #56 from hawesie/hydro-devel

Reconnecting on nav start and returning empty responses correctly.
Added scaling for nav timeout.
Catching killer exception.
REmoved asserts for long-term stability.
fixing preemption mechanism
making sure goals are cancelled down nav pipeline
Merge pull request #58 from BFALacerda/hydro-devel

cancelling action properly
making sure robot gets to the correct pose on goal waypoint
replanning added for unexpected state transitions
handling situations where no message is published in /current_node
Output changes
Changes for local testing.
Ensuring mdp planner shuts down when asked.
Updated test executor to match mdp expectations, so now uses monitored navigation.
Added a stricter cancel for navigation and execution.

This new version does not wait to receive a callback from the cancelled action server. This is dangerous in that the next task may start while the previous task is still ending, but there isn't a huge problem with this in our current tasks. A better solution would be to wait a bit, then give up on waiting for the callback, but this is hard in the current design. Probably needs to be reimplemented as a state machine to make this cleaner.