Issue3 add GET routes

Fixes #3

Completes function GET endpoints based documented details in wiki:
https://github.com/DoSomething/mb-logging-api/wiki

- GET /api
- GET /api/v1
- GET /api/v1/imports/:start date/:end date
- GET /api/v1/imports/summaries/:start_date/:end_date
`start_date` and `end_date ` are assigned based on the URL that's posted to based on the route:
```
app.get('/api/v1/imports/summaries/:start_date/:end_date', function(req, res) {
```
I don't think it's possible to get to `UserImportSummary.prototype.get` without the values being assigned. This does point out the need to validate the values to resolve to a valid date.