Add Android physic buttons input detection

- Added functions for detect when pressed, during down and when
released.
- Added defines for back, menu, volume up and down button numbers.

src/raylib.h

@@ -186,6 +186,12 @@
 
 // TODO: Review Xbox360 USB Controller Buttons
 
+// Android Physic Buttons
+#define ANDROID_BACK            4
+#define ANDROID_MENU            82
+#define ANDROID_VOLUME_UP       24
+#define ANDROID_VOLUME_DOWN     25
+
 // Some Basic Colors
 // NOTE: Custom raylib color palette for amazing visuals on WHITE background
 #define LIGHTGRAY  (Color){ 200, 200, 200, 255 }   // Light Gray
@@ -581,6 +587,9 @@ bool IsGamepadButtonUp(int gamepad, int button);        // Detect if a gamepad b
 int GetTouchX(void);                                    // Returns touch position X (relative to screen size)
 int GetTouchY(void);                                    // Returns touch position Y (relative to screen size)
 Vector2 GetTouchPosition(void);                         // Returns touch position XY (relative to screen size)
+bool IsButtonPressed(int button);                       // Detect if an android physic button has been pressed
+bool IsButtonDown(int button);                          // Detect if an android physic button is being pressed
+bool IsButtonReleased(int button);                      // Detect if an android physic button has been released    
 
 //------------------------------------------------------------------------------------
 // Gestures and Touch Handling Functions (Module: gestures)