[atlas] measurement start_time and stop_time fields

I think that there might be some confusion here as to the use of the 
`use_iso_time` and `format` parameters, so I'd like to clear those up 
now.  I've also patched the code to do what you need though, but one 
thing at a time.

The `use_iso_time` flag *only changes the response* to use ISO 
timestamps rather than UNIX timestamps (the default).  Additionally, as 
UNIX timestamps are the default, setting `use_iso_time=false` does 
nothing for you here.

The `format` parameter is only useful for when you're fetching 
measurement results and not when you're requesting measurement 
metadata.  The `=txt` stuff is a hack we built on top of standard REST 
practises in deference to the fact that result datasets can be very big 
and near impossible to parse as single JSON units.  For all of the 
other calls, the API follows the same path as you'll see with other 
REST environments: JSON and XML.  You can access these formats by using 
`format=json` and `format=xml` respectively.

As for actually filtering your metadata by `start_time` and `stop_time` 
using UNIX timestamps instead of ISO ones, this was a limitation of the 
framework we're using.  I've since burned a couple hours today 
overriding the default behaviour to allow for using *either* a UNIX 
timestamp or an ISO one.  Go ahead and give it a spin.  Let me know if 
it works as expected.