add .unixtime(), .milliseconds()

[calvinfo]

So many of these guys require odd date formats for their values. Might be worth adding methods to the prototype indicating that we should convert any object values which match an iso8601 string to unix timestamps

CustomerIO = integration('Customer.io')
  .mapper(mapper) // unix timestamps get passed in here
  .retries(2)
  .unixtime()

cc @yields

[yields]

sounds good :)