[Bug 799693] Zero fill data

Add support to fill data with zeros, as well as expanding the data to fill the request date range (so if you ask for 10 days of data when there is only 5 days available, it will still show 10 days worth of graph).

In doing this, I decided that it would be simpler if there were always start and end dates, instead of allowing them to overflow to +/- infinity. To accomplish this I changed the default view to 1 week, instead of all data. Because of this I also had to modify the tests to work with relative dates, instead of absolute dates.

If the end date is left off, it defaults to today. If the start date is left off, it defaults to the end date minus 7 days.