First draft of sensor interfaces.

- Force/torque (wrench)
- Orientation
- IMU (very crude approximation)
Add sensor reference frame field.
Add sensor ref frame field and capability queries.
Merge branch 'master' into sensor_interfaces
Scrape orientation interface prototype.
Explicitly initialize IMU sensor handle members.
Remove Eigen dependency from hardware_interface.

- Expose force-torque and IMU sensor data as const pointers to the raw data.

- Client code should wrap raw data however they prefer.
Merge branch 'master' into sensor_interfaces
Merge branch 'master' into sensor_interfaces
Merge branch 'master' into sensor_interfaces
Update sensor interfaces implementation.

- Use resource managing classes introduced in recent hardware interface rework.

- Conform to unified public API.
Merge branch 'master' into sensor_interfaces
Add missing brace.
Merge branch 'master' into sensor_interfaces
Revert "Fixed PLUGINLIB_DECLARE_CLASS deprecated errors"

This reverts commit cd9aba265a380bafebb70d63081405d857e9380d.
Merge branch 'fuerte_backport' into sensor_interfaces
Add default constructors to sensor handles.
Unit test sensor interfaces.
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.