Unmark as invalid when token can be refreshed

It is totally possible that the server (in a bad state) responds temporarily with an "invalid x" and on a retry actually refreshes the token. So we need to reset the flag ´self.invalid´.
Merge pull request #29 from kaste/patch-1

Unmark as invalid when token can be refreshed
Handle PKCS12 unicode passwords

The load_pkcs12 can't handle passing unicode objects. Instead, encode the given 
string as bytes before passing to the function.
Merge pull request #31 from jterrace/master

Handle PKCS12 unicode passwords