API Metric value aggregation

[amorphic]

Add an optional parameter to perform aggregation on metric values returned from the API.

Currently the API returns all values within a requested time period. This is often a higher resolution than is necessary, as for the new graphs discussed in #7.

Allow the user to add something like aggregation_secs=3600 to get datapoints aggregated hourly.

Also specify aggregation method with a separate parameter. Something like aggregation_method=avg (min/max/avg/mean with avg being default).