Add removeAll to preferences (#987)

This removes all keys except for .type.

wpilibj/src/main/java/edu/wpi/first/wpilibj/Preferences.java

@@ -173,6 +173,17 @@ public class Preferences {
     m_table.delete(key);
   }
 
+  /**
+   * Remove all preferences.
+   */
+  public void removeAll() {
+    for (String key : m_table.getKeys()) {
+      if (!".type".equals(key)) {
+        remove(key);
+      }
+    }
+  }
+
   /**
    * Returns the string at the given key. If this table does not have a value for that position,
    * then the given backup value will be returned.