revelation2pass: add plain XML import

I found that revelatio2pass.py script doesn't work. It can not decrypt
my password file. I got following error message:

raceback (most recent call last):
 File "git/password-store/contrib/importers/revelation2pass.py", line 159, in <module>
   main(args.FILE, verbose=args.verbose, xml=args.xml)
 File "git/password-store/contrib/importers/revelation2pass.py", line 140, in main
   cleardata_gz = decrypt_gz(password, data)
 File "git/password-store/contrib/importers/revelation2pass.py", line 117, in decrypt_gz
   ct = c.decrypt(cipher_text[28:])
 File "/usr/lib/python2.7/site-packages/Crypto/Cipher/blockalgo.py", line 295, in decrypt
   return self._cipher.decrypt(ciphertext)

I was unable to fix the problem, but I created a workaround, that add
plain XML import option to the revelation2pass.py script. Revelation can
export its password file as plain XML format.