Take clock skew and min validity into consideration

[klippx]

Real implementation of #390 (clock skew)

Changes

• Read issued at from the actual token instead of using Time.now (client time)
• Read expires_at from the actual token instead of using Time.now (client time) and adding expires_in to it
• Take clock skew into consideration when checking if token is expired

In case the token is not a JWT token with the expected keys, the old functionality is intact (use Time.now)

Thanks to @Ashaeron for helping me :)

[coveralls]

Pull Request Test Coverage Report for Build 645

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at ?%

---

Totals
Change from base Build 625:	0.0%
Covered Lines:
Relevant Lines:	0

---

💛 - Coveralls

[klippx]

Actually, I don't believe this is right. I need to think more about this...