Added a conditional command that chooses 1 of 2 commands (#435)

Tests and documentation for ConditionalCommand were also added. This is a fixed version of #9, so it doesn't support lambdas.

Closes #9

wpilibcIntegrationTests/src/command/MockConditionalCommand.cpp

@@ -0,0 +1,20 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2017. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "command/MockConditionalCommand.h"
+
+using namespace frc;
+
+MockConditionalCommand::MockConditionalCommand(MockCommand* onTrue,
+                                               MockCommand* onFalse)
+    : ConditionalCommand(onTrue, onFalse) {}
+
+void MockConditionalCommand::SetCondition(bool condition) {
+  m_condition = condition;
+}
+
+bool MockConditionalCommand::Condition() { return m_condition; }