adds support for timestamp

[hwhelchel]

[coveralls]

Coverage remained the same at 100.0% when pulling 3b8d1b2255c3e61d64b4a4b462ac958830688c2b on hwhelchel:add-timestamp-to-track into 0f0a35389eb824e29c1ee099661eb3823653be42 on heap:master.

[thatandyrose]

@pwnall would be great to get this merged! 😍

[pwnall]

@thatandyrose I am not a collaborator on this repo anymore. Perhaps @enochcheung can help?

The change LGTM, with the nit that I'd prefer "ISO 8601" in strings.

[talos]

@hwhelchel According to Travis this change breaks tests in Ruby 1.8 through 2.0, as well as jruby-9. This would need to be resolved before we can merge (which we would like to do!)