Restore support in ApplyExternalWrench for non-composite models

Fixes #131. Still not the optimal solution.
Fix POSITION_DIRECT control mode
Remove trailing whitespaces
[controlboard] streamline: always use setControlMode for changing control mode
[controlboard] fix setPositions methods
[controlboard] streamline implementation of set*Mode* methods

Move all setter method of control modes to use the two basic
calls setControlMode and setInteractionMode
[controlboard] Document unit of measuements of class attributes
Fix typo in setPosition implementation
Add proper reset of reference values when changing control mode, fixes #129
only reset pid set values on actual switching, and only switch to known control modes
Fixed some spaces
Merge pull request #138 from robotology/fixPositionDirect

Fix position direct and improve compliance with yarp controlboard specification
Merge branch 'master' into development

Conflicts:
	src/yarp_drivers/ControlBoardDriverPositionControl.cpp