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/shared/include/Commands/PIDCommand.h

@@ -15,6 +15,8 @@
 #include "PIDOutput.h"
 #include "PIDSource.h"
 
+namespace frc {
+
 class PIDCommand : public Command, public PIDOutput, public PIDSource {
  public:
   PIDCommand(const std::string& name, double p, double i, double d);
@@ -55,3 +57,5 @@ class PIDCommand : public Command, public PIDOutput, public PIDSource {
   void InitTable(std::shared_ptr<ITable> subtable) override;
   std::string GetSmartDashboardType() const override;
 };
+
+}  // namespace frc