Add sleep and gpio support + examples

Added sleep support for MCP2515 + MCP2551. This way, you can reduce the power consumption of a complete node down to around 240uA!
Changes:
- Reintroduced gpio support from commits 43521de/bac66d1 (adlerweb, 15/01/2018) and corrected the errors
- Introduced advanced sleep support based on https://github.com/coryjfowler/MCP_CAN_lib/pull/10/files
- added example code (receive_sleep and send_sleep)
- tested everything

examples/gpioRead/gpioRead.ino

@@ -19,7 +19,7 @@ void setup()
     }
     Serial.println("CAN init ok");
 
-    if(CAN.pinMode(MCP_TX2RTS, MCP_PIN_IN))
+    if(CAN.mcpPinMode(MCP_TX2RTS, MCP_PIN_IN))
     {
         Serial.println("TX2RTS is now an input");
     }
@@ -32,7 +32,7 @@ void setup()
 void loop()
 {
     Serial.print("TX2RTS is currently ");
-    Serial.println(CAN.digitalRead(MCP_TX2RTS));
+    Serial.println(CAN.mcpDigitalRead(MCP_TX2RTS));
     delay(500);
 }