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

wpilibjIntegrationTests/src/main/java/edu/wpi/first/wpilibj/command/CommandTestSuite.java

@@ -19,7 +19,7 @@ import edu.wpi.first.wpilibj.test.AbstractTestSuite;
 @RunWith(Suite.class)
 @SuiteClasses({ButtonTest.class, CommandParallelGroupTest.class, CommandScheduleTest.class,
     CommandSequentialGroupTest.class, CommandSupersedeTest.class, CommandTimeoutTest.class,
-    DefaultCommandTest.class})
+    ConditionalCommandTest.class, DefaultCommandTest.class})
 public class CommandTestSuite extends AbstractTestSuite {
 
 }