[security] improve token generator.

This is not ok. base_convert only supports up to MAX_INT-1. I recommend using base64_encode and trimming = on the right.. If the token also can be used inside the uri, then uri safe base64 encoding should be done...

Apply comments done: https://github.com/Payum/Payum/commit/59748392d233783140547ca1240dfae61202a0c6#commitcomment-4667875
Merge pull request #90 from Payum/improve-token-generation

[security] improve token generator.
Merge branch '0.6'