Use individual cmake files for each namespaces

[jslee02]

This pull request divides the single cmake file into multiple files for each namespace. DART have some optional dependencies such as bullet collision, nlopt, ipopt, and snopt, and there are optional code, which should be built only when the corresponding optional dependencies are installed. Previously, DART has used single cmake files to build dart and dart-core (not examples nor unittests) and it makes hard to control the optional code to be built depends on the corresponding dependencies.

Additionally, this pull request adds all-inclusive headers, dart.h and dart-core.h. We had to include all the necessary header files of dart to write a software that has dependency of dart. For example, this is part of source code of hardcodedDesign app in dart:

-#include "dart/collision/CollisionDetector.h"
-#include "dart/dynamics/Skeleton.h"
-#include "dart/dynamics/BodyNode.h"
-//#include "dart/constraint/OldConstraintDynamics.h"
-#include "dart/dynamics/Joint.h"
-#include "dart/dynamics/MeshShape.h"
-#include "dart/dynamics/BoxShape.h"
-#include "dart/dynamics/CylinderShape.h"
-#include "dart/dynamics/EllipsoidShape.h"
-#include "dart/utils/Paths.h"
-#include "dart/common/Timer.h"
-#include "dart/math/Helpers.h"
-#include "dart/gui/GLFuncs.h"
-#include "dart/gui/Win3D.h"
-
-namespace dart {
-namespace dynamics {
-class Skeleton;
-class ConstraintDynamics;
-}  // namespace dynamics
-}  // namespace dart

This number of lines can be reduced single line as:

#include "dart/dart.h"

or

#include "dart/dart-core.h"

If we want to include specific namespaces of dart then:

#include "dart/common/common.h"
#include "dart/math/math.h"
#include "dart/dynamics/dynamics.h"

This fixes #158

[jslee02]

+1

[karenliu]

+1