base_local_planner: adds waitForTransform

The first waitForTransform (Line 107 - 112) resolves #196. I also added the second one to avoid similiar issues in other situations.

An alternative to using `waitForTransform` could be requesting the latest available transform (`ros::Time(0)`). If applicable, I generally prefer this alternative, but since I'm not too familiar with the various parts of the navi stack, I decided to stick as close to the current code and logic as possible.