Allow additional timestamp fields other than `timestamp`

[orweinberger]

Currently, it's not possible to push another valid timestamp date into Joola, this is probably due to some specific date handling Joola does on the timestamp field, but not on other time stamped fields.

For example, here's how the timestamp field looks like after it's been pushed to mongo:

"timestamp" : ISODate("2010-04-21T15:10:14Z"),

And here's another timestamp field that was pushed as new Date().toISOString():

"test_timestamp" : "2010-04-21T15:10:14Z",