Implement a continuouse replanning option

[spuetz]

For some cases / applications / scenarios it could be useful to have a continuous replanning option. This could also solve #223.

[corot]

move_base action already have continuous replanning.
When using you own executive, you can just call get_path at the rate you want

[spuetz]

I meant without calling the GetPath action all the time, thus the planner can trigger plan updates. I explicitly mean path planning methods like LPA*, D* Lite, and Field D*.