Add support in mongodb for choosing any integer field as the 'time' field

[rameshvk]

Yo, check this out and LMK what you think (feel free to not take it if this doesn't fit your general direction). I needed this change (a lot of the user actions records have other fields, most importantly createdTime -- now I can view if there are user behavior changes over time by plotting actions against created time). I could have split the actions out but this is easier..

Ideally though, it would be nice if the views were a bit more decomposed into specifying x-axis and y-axis independently (i.e. I should be able to specify any integer field as X-axis, I should be able to specify any integer field for the aggregation, I should be able to specify any set of fields for the Y-axis -- non-integer fields should be grouped and integer fields should be bucketed). Thinking in these terms, the fields can also take simple functions (i.e. log(time) can be the X axis in which case the bucket is applied on the log too).

[okayzed]

awesome! i ported it to postgres and added the option to the area view. in general, i think this could be an option available on all views (it does make sense for all time filters), but it's an advanced feature.