marshalling and unmarhalling of datetime

Python datetime objects can be given in parameters that are converted to UTC time and stored
having millisecond precision

while marshalling and unmarshalling the datetime. Works with local datetime (i.e. converts them to utc
while storing and parses them accordingly). Need to test with timezone-aware datetimes

Also, write tests!
added marshalling tests for datetime

- may need to add for other types too
- Added comment describing behaviour of datetime marshalling
lint fixes
added iso8601 in requirements.txt
descriptive comments
lint fixes
seperate tests and bumped version
Merge pull request #13 from rackerlabs/timestamp_marshal

marshalling and unmarhalling of datetime