Abstract tag manipulation functions.

- Factorize Mifare*Tag as MifareTag;
  - Factorize mifare_*_get_tags() / mifare_*_free_tags() as freefare_get_tags() and freefare_free_tags();
  - Add a new freefare_get_tag_type() function to get a tag type;
  - Update regression test suite;
  - Update example.

While this is a major change that basically change all the API, programs using libfreefare should be easily modified by replacing any Mifare*Tag variable by a generic MifareTag one, adding a few lines of code to check the target's type using freefare_get_tag_type(), and changing any call to mifare_*_get_tags() / mifare_*_free_tags() by the generic freefare_get_tags() and freefare_free_tags() functions.

test/mifare_ultralight_fixture.h

@@ -17,4 +17,4 @@
  * $Id$
  */
 
-extern MifareUltralightTag tag;
+extern MifareTag tag;