Trivial doc fix.
Harmonize MultiDofSegment constructor signature.

Segment implementations should have a similar way to be constructed, namely
by specifying four parameters:

  start_time, start_state, end_time, end_state

The types of these parameters depends on the segment type, but the structure
is preserved.
Add a non-iterator based findSegment(...) overload.

Used for convenience purposes.
Add utilities for reading trajectories from ROS msgs.
Move trajectory_interface to a separate directory.

General-purpose trajectory_interface functionality should live in a separate
place (ie. ROS package) than the joint_trajectory_controller. This is a first
step towards this separation.
Restructure error reporting log statements.
Scope init method as protected.
Add new trajectory segment type.

- Multi-dof quintic spline segment that can be constructed from ROS message
  data structures.
Implement trajectory init from ROS message data.

Update unit tests accordingly.