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/interfaces/Accelerometer.h

@@ -7,6 +7,8 @@
 
 #pragma once
 
+namespace frc {
+
 /**
  * Interface for 3-axis accelerometers
  */
@@ -45,3 +47,5 @@ class Accelerometer {
    */
   virtual double GetZ() = 0;
 };
+
+}  // namespace frc

wpilibc/shared/include/interfaces/Gyro.h

@@ -7,6 +7,8 @@
 
 #pragma once
 
+namespace frc {
+
 /**
  * Interface for yaw rate gyros
  */
@@ -54,3 +56,5 @@ class Gyro {
    */
   virtual double GetRate() const = 0;
 };
+
+}  // namespace frc

wpilibc/shared/include/interfaces/Potentiometer.h

@@ -9,6 +9,8 @@
 
 #include "PIDSource.h"
 
+namespace frc {
+
 /**
  * Interface for potentiometers.
  */
@@ -25,3 +27,5 @@ class Potentiometer : public PIDSource {
 
   void SetPIDSourceType(PIDSourceType pidSource) override;
 };
+
+}  // namespace frc