Add 4800 baud rate. Closes #20.

gps_uart.h

@@ -5,8 +5,8 @@
 
 #define RX_BUF_SIZE 1024
 
-static const int gps_baudrates[5] = {9600, 19200, 38400, 57600, 115200};
-static int current_gps_baudrate = 0;
+static const int gps_baudrates[6] = {4800, 9600, 19200, 38400, 57600, 115200};
+static int current_gps_baudrate = 1;
 
 typedef struct {
     bool valid;