Handle first waypoints with time_from_start == 0.

Two different scenarios have been addressed when the first waypoint has zero
time_from_start:

- Bugfix: If the approach trajectory is null, but non-planning joints move
  a reach time must be computed for the first waypoint.

- Cosmetic fix: Current state coincides with first waypoint. We make the
  time_from_start be a small, almost zero value to prevent the controllers
  from issuing a warning.
Don't compute approach time if specified.

When skip_planning is set to True, the input motion might specify a valid
time_from_start for the first waypoint. The automatic reach time computation
is thus done only when the first waypoint contains a zero time_from_start.
First iteration of move_joint script. Refs #33.
Silence compiler warnings.