Respect exp claim when provided in create()

[typerandom]

Fix so that the exp claim is used if provided when calling create().

How to verify

Test it with the following code:

var nJwt = require('njwt');

var fakeSecret = 'd8b148ae-917e-4ec3-bb46-f36718ac5cde';
var currentEpochTime = Math.floor(new Date().getTime() / 1000);
var twoHoursFromNowInSeconds = currentEpochTime + (2 * 60 * 60);

var testOptions = {
  exp: twoHoursFromNowInSeconds * 1000 // As milliseconds.
};

var jwt = nJwt.create(testOptions, fakeSecret);

console.log('%s should be the same as %s', jwt.body.exp, twoHoursFromNowInSeconds);

Fixes #23

[coveralls]

Coverage remained the same at 100.0% when pulling c63440e on fix-create-should-respect-exp-claim into 9e177c3 on master.