use iso8601 to parse datetime of azure storage entity

In docs for [Python 2.7.5](https://docs.python.org/2.7/library/datetime.html#datetime-objects) and [Python 3.4.1](https://docs.python.org/3.4/library/datetime.html#datetime-objects) it is said that

`class datetime.datetime(year, month, day[, hour[, minute[, second[, microsecond[, tzinfo]]]]])`
>`0 <= microsecond < 1000000`
>If an argument outside those ranges is given, *ValueError* is raised.

The main thing here, is that *microsecond* shouldn't be greater than 1000000

I was trying to access entities, that were saved into storage by back-end written in C# where DateTime property was set with `DateTime.UtcNow.ToString("o");` value. So the resolution down to 1/10^7 of a second

And with that 1/10^7 of a second, I got *ValueError*:
`ValueError: time data '2014-09-04T22:11:05.5269341Z' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'`

With iso8601 module everything worked like a charm since there is no such limitations for microseconds part.