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/device/serial_comm.rs

@@ -0,0 +1,12 @@
+use super::Result;
+
+#[cfg(any(feature = "serialport_comm", feature = "ftdi_comm"))]
+pub const DEFAULT_BAUD_RATE: u32 = 38_400;
+
+/// An API to communicate with a serial device
+pub trait SerialComm {
+    fn write_all(&mut self, data: &[u8]) -> Result<()>;
+    fn read(&mut self, data: &mut [u8]) -> Result<usize>;
+    fn set_baud_rate(&mut self, baud_rate: u32) -> Result<()>;
+    fn purge_buffers(&mut self) -> Result<()>;
+}