Add nfc_register_driver. Issue 137.

The nfc_register_driver allows users of the library to write their
own device drivers without needing to resort to hacking libnfc itself.

include/nfc/nfc-types.h

@@ -42,6 +42,11 @@ typedef struct nfc_context nfc_context;
  */
 typedef struct nfc_device nfc_device;
 
+/**
+ * NFC device driver
+ */
+typedef struct nfc_driver nfc_driver;
+
 /**
  * Connection string
  */