Miscellaneous cleanups for HAL, wpilibc, and wpilibj JNI (#589)

* Static functions in the HAL implementation were placed in the hal namespace
* "using namespace" declarations in HAL/cpp/Log.h and Timer.cpp were replaced
  with "using" declarations for std::chrono
* An extra include was removed from AnalogGyro.cpp
* InterruptableSensorBase's constructor was defaulted
* Newlines were added to some wpilibc integration tests for grouping
* A variable in HALUtil.h was renamed to follow the style guide

Supersedes #586

hal/lib/athena/Accelerometer.cpp

@@ -77,6 +77,8 @@ enum Register {
   kReg_OffZ = 0x31
 };
 
+namespace hal {
+
 static void writeRegister(Register reg, uint8_t data);
 static uint8_t readRegister(Register reg);
 
@@ -182,6 +184,8 @@ static double unpackAxis(int16_t raw) {
   }
 }
 
+}  // namespace hal
+
 extern "C" {
 
 /**