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.

wpilibcIntegrationTests/src/command/CommandTest.cpp

@@ -14,6 +14,8 @@
 #include "command/MockCommand.h"
 #include "gtest/gtest.h"
 
+using namespace frc;
+
 class CommandTest : public testing::Test {
  protected:
   void SetUp() override {

wpilibcIntegrationTests/src/command/MockCommand.cpp

@@ -7,6 +7,8 @@
 
 #include "command/MockCommand.h"
 
+using namespace frc;
+
 MockCommand::MockCommand() {
   m_initializeCount = 0;
   m_executeCount = 0;