Add support for an averaged output

This pull request adds support for an aggregated display of the memory usage which is helpful for not just finding which action is responsible for the largest increases, but which action is responsible for the most total memory increase. It adds a section that looks like this:

```
  Aggregated Totals:
  Action                                  Max     Mean    Min     Total   Number of requests
  SportsController#show                   101560  19754   4       5590540 283
  CommentsController#create               8344    701     4       253324  361
  ColorSchemesController#show             10124   739     4       68756   93
  PublicPagesController#join              9004    1346    8       51172   38
  DashboardsController#show               13696   2047    8       45036   22
  SessionsController#create               9220    528     8       17448   33
  GroupsController#show                   10748   1314    8       15776   12
```