Add --prefix option to rosrun

With catkin, binaries have moved from the bin/ folder within the package into the devel/build folders in the catkin workspace; this makes it difficult to find them without understanding the workspace layout.

I've also seen a number of comments indicating that "gdb doesn't work with rosrun"

I think the solution to these is that rosrun should have a `--prefix` option to allow users to run their programs within gdb, valgrind and other debug tools.