Adds jobs to crontab using backdrop-collector library

This commit introduces the backdrop-collector library for adding jobs to the
crontab in a similar way to backdrop-ga-realtime-collector.

1. Rename `backdrop.collector` package to `collector` to avoid conflict with backdrop-collector
2. Create a `collect.py` script as expected by `backdrop-collector`
3. Add post deploy hook to create the crontab in the same way as ga realtime
Move default period start/end into period_range
Use sensible exit statuses
Move query logging to query_ga function
Add test for applying mappings to documents
Refactor tests to use has_entries

has_entries seemed clearer than multiple has_entry calls.
Refactor send_records_for and add tests

Pull out functions for querying over a date range (query_for_range) and
building a set of documents (build_document_set).
Merge pull request #10 from alphagov/use-backdrop-collector

Adds jobs to crontab using backdrop-collector library