Add X-Auth-Token-Lifetime and user_set_tokenlifetime option 

Hello,

first thank your very much for your work on swauth! 

I just added an configuration option to swauth which allows users to set the token lifetime when requesting a new one. It's off by default and can be enabled with the option user_set_tokenlifetime in the proxy conf. 

Some of our users need tokens with a longer lifetime because they use them in batch jobs. 

Best Regards,

Christian