Add initial version of safety_limits_interface

- Soft limits for position and effort interfaces, based on existing PR2 safety
  controllers.

- Saturation limits for velocity interfaces.

- Unit tests for position and velocity interfaces, not yet for effort.

- Dependency on URDF package. Should be removed.
Separate URDF dependencies from interface core.

- Create URDF-agnostic joint limits specification.

- Add convenience methods for populating these structs from URDF joints.
  This might be moved elsewhere in the future, maybe to a place where all
  URDF-specific stuff lives.

- Update tests.
Handle acceleration limits for velocity joints.

- Acceleration limits now affect the maximum allowed command for
  velocity-controlled joints.
Maintenance: Test class names reorganization.
Get rid of base class for joint limits handles.
Extensive file, namespace, class renaming.
Add documentation.
Minor maintenance fixes.
Add max_jerk to limits specification.
Add joint limits parsing from rosparam + unit test.
Add .gitignore
Allow unsetting limits specification from rosparam.

- Update tests.
Lower severity of log message.
Add attribution for soft_limits code.

- Soft-limits enforcing is based on a previous implementation by Willow Garage.
  Add them in the copyright holders list.
Remove temporary file from version control.
Merge pull request #76 from pal-robotics/joint_limits_interface

Joint limits interface