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/Error.h

@@ -20,6 +20,8 @@
 #undef GetMessage
 #endif
 
+namespace frc {
+
 //  Forward declarations
 class ErrorBase;
 
@@ -59,3 +61,5 @@ class Error {
   const ErrorBase* m_originatingObject = nullptr;
   double m_timestamp = 0.0;
 };
+
+}  // namespace frc