Add plugin architecture

This pull request adds a new plugin mechanism.

It currently depends on the `backdrop-collector-plugins` package, which
hasn't yet been reviewed.

The basic idea is that it should be possible to perform arbitrary
transformations to the documents before they are stamped with a `humanId`.

These arbitrary transformations may perform an aggregation on the input
and so need to see all input documents and specify the complete output
document set.

```json
{
  "dataType": "content_dashboard_visitors_count",
  "query": {
    "id": "ga:123456",
    "metrics": [
      "visitors"
    ],
    "dimensions": [
      "customVarValue9"
    ]
  },
  "target": {
    "url": "http://localhost:3039/data/foo/visitors-count",
    "token": "foo-bearer-token"
  },
  "plugins": [
    "ComputeDepartmentKey('customVarValue9')",
    "RemoveKey('customVarValue9')",
    "AggregateKey(aggregate_count('visitors'))"
  ]
}
```

The above query configuration shows the new "plugins" configuration
option. If "plugins" is specified, then `backdrop.collector.plugins` is
required to run.

In the example above, plugins are run in turn on all documents, in the order
they are provided. `ComputeDepartmentKey` computes a new `department`
parameter for each document based on the content of `customVarValue9`,
`RemoveKey` deletes the old `customVarValue9`. Finally, because there was
a many-to-one mapping of `customVarValue9` to `department`, the `visitors`
count needs to be summed for documents where all values except `visitors`
are equal. This is achieved through `AggregateKey`, which is capable
of aggregating multiple fields as well as different types of fields.
`aggregate_count` in this case specifies that the aggreagtion should happen
using the `sum` builtin. `aggregate_rate('rate_var', 'count_var')` allows
aggregation of percentages, weighted by counts.

Plugin strings are python expressions evaluated in the
[`backdrop.collector.plugins`](https://github.com/scraperwiki/backdrop-collector-plugins/blob/master/backdrop/collector/plugins/__init__.py) namespace.