Added Window to dictionary text/thrift formats

After this, shit is going to get real.
Does it make sense to support granularities less than a day, e.g. hour? I don't think minute and second makes a lo to of sense.

I'm not sure sure to what extent `Window` will be used ... but would it be used for a feature such as comparing the number of events that occur 00:00-12:00 vs 12:00:24:00, over a window of the last 3 weeks. Would `Window` be used to capture the 00:00-12:00 part?