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

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 <time> 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).