Remove "using namespace std" from headers. Fix issue #73.
Fix #4. Rename M_EPSILON to DART_EPSILON to prevent confliction with external M_EPSILON.
Fix compiler error caused by last commit
Validation code is fixed in BodyNode.cpp for debug mode.
Use Featherstone forward dynamics algorithm.
Add BodyNode::setExtForce(...) and BodyNode::setExtTorque(...).
Add default value for function parameter for BodyNode::setExtTorque(...) and BodyNode::addExtTorque(...)