Set ttl in wrap

[nguyenchr]

As discussed in #12

This allows different ttl values to be set per function that is being wrapped

For example

var getShortLivedField = function(id, callback) {
  ...
}

var getLongLivedField = function(id, callback) {
  ...
}

app.get('/short/lived', function(req, res) {
  var cache_key = "short:" + req.params.id;
  var ttl = 1;
  cache.wrap(cache_key, function(cache_cb) {
    getShortLivedField(req.params.id, cache_cb);
  }, ttl, function(err,result){
    res.send(result);
  });
})

app.get('/long/lived', function(req, res) {
  var cache_key = "long:" + req.params.id;
  var ttl = 600;
  cache.wrap(cache_key, function(cache_cb) {
    getLongLivedField(req.params.id, cache_cb);
  }, ttl, function(err,result){
    res.send(result);
  });
})

ttl is optional in wrap(), if it is not passed in then cache.set will be called with a ttl of undefined

[coveralls]

Coverage remained the same when pulling 0f685c5 on nguyenchr:set-ttl-in-wrap into 1d352ed on BryanDonovan:master.

[BryanDonovan]

Thanks, I'll cut a new release version asap

[BryanDonovan]

This is merged into 0.14.0. Thanks again!