Serialport Support

- allow communication to elm327 device via a serialport
- allow creation of elm327 device without requiring unwrap()
- added some extra documentation
- expose reset() functionality of device
- serial_comm and ftdi_comm abstractions added

src/commands/types.rs

@@ -42,7 +42,7 @@ impl From<u16> for Dtc {
             1 => Dtc::Chassis(n),
             2 => Dtc::Body(n),
             3 => Dtc::Network(n),
-            _ => unreachable!(),
+            _ => unreachable!(), // can't happen, only two bits
         }
     }
 }