Parse expires_in to future timestamp

[jeffutter]

This forces both string and integer values for expires in to parse to an
integer that is added to unix_now, while leaving expires to simply be
parsed to an integer.

Fixes #58

[coveralls]

Coverage decreased (-1.2%) to 89.759% when pulling 7e0daca on jeffutter:string_expires_in into c034793 on scrogson:master.

[coveralls]

Coverage decreased (-1.2%) to 89.759% when pulling eb09c5f on jeffutter:string_expires_in into c034793 on scrogson:master.

[scrogson]

Awesome! Thank you! 👍