Add support for leeway

[dgrijalva]

It's been requested.

See #86

[dgrijalva]

The RFC says

Implementers MAY provide for some small leeway, usually no more than a few minutes, to account for clock skew

This suggests to me this is not meant to be a property of the token, but an option to the parser. I think we can solve this in the following way:

• Add a new struct type ValidationOptions with one property (for now) Leeway int
• Add ValidationOptions as a property on Parser
• Modify Valid() on Claims to be Valid(opts ValidationOptions)

This would allow leeway to be supported with only very small compatibility changes. It would also allow for future extensions/options without breaking API compatibility.

[dgrijalva]

In the meantime, you an solve this problem for yourself using the following:

type MyClaims struct {
  *StandardClaims
  Foo string
}

func (c * MyClaims) VerifyExpiresAt(cmp int64, req bool) bool {
        var leeway = 120 // two minutes
    return c.StandardClaims.VerifyExpiresAt(c.ExpiresAt, cmp - leeway, req)
}

[dgrijalva]

This ticket replaced by #139