Namespace all wpilibc functions/classes into "frc" namespace. (#311)

Base.h provides a backwards compatibility shim (enabled unless
NAMESPACED_WPILIB is defined) that does a "using namespace frc".
However, as some header files do not include Base.h, this may
be a breaking change in some corner cases (with an easy fix).

Fixes #218.

wpilibc/sim/include/simulation/MainNode.h

@@ -16,6 +16,8 @@
 
 using namespace gazebo;
 
+namespace frc {
+
 class MainNode {
  public:
   static MainNode* GetInstance() {
@@ -61,3 +63,5 @@ class MainNode {
     }
   }
 };
+
+}  // namespace frc