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/src/interfaces/Potentiometer.cpp

@@ -9,6 +9,8 @@
 
 #include <Utility.h>
 
+using namespace frc;
+
 void Potentiometer::SetPIDSourceType(PIDSourceType pidSource) {
   if (wpi_assert(pidSource == PIDSourceType::kDisplacement)) {
     m_pidSource = pidSource;