Issue17 summery details logging

** NOTE**: In development, not ready for merge. For discussion only.

Adds endpoint: `/api/v1/imports/summaries/existing` to support logging of existing stats as addition to entries in `db['import-summaries']`, indexed on `target_CSV_file`.

**To test**:
- start`mongod` and $node mb-logging-api-server`.
- Test http POST to `http://localhost:4733/api/v1/imports/summaries/existing?type=user_import&source=niche.com&target_CSV_file=2014-09-10.csv` 
```
with parameters:
  - `mailchimp`
  - `drupal`
    - `mobile_commons` -> `undeliverable`
    - `mobile_commons` -> `existing`
    - `mobile_commons` -> `no_subscription`
    - `mobile_commons` -> `other`
    - `mobile_commons` -> `total`
  - `total`

to populate fields in `import-summary` document:
```
    existing : {
      mailchimp : { type : Number },
      drupal : { type : Number },
      mobile_commons : {
        undeliverable : { type : Number },
        existing : { type : Number },
        no_subscription : { type : Number },
        other : { type : Number },
        total : { type : Number }
      },
      total : { type : Number }
    },
```
- update of entry based on `target_CSV_file` value.