Add period and dimension values to id

Without these the id is not unique. Base64 encoding the id to allow any
dimension values.

@maxfliri
@robyoung
Bump gapy to 0.0.6

Failed when no rows are returned.
Update the default start / end dates

Both should be 1 week ago as we're just dealing with weekly data. When
we handle other periods this should depend on the period.
Allow dimension keys to be mapped to new keys
Date parsing revised + more logging

@maxfliri
Merge pull request #3 from alphagov/add-period-and-dimensions-to-id

Add period and dimension values to id
Merge pull request #4 from alphagov/bump-gapy-to-0.0.6

Bump gapy to 0.0.6
Merge branch 'master' into update-default-start-end

Conflicts:
	backdrop/collector/ga.py
Merge branch 'master' into allow-mapping-of-dimension-names

Conflicts:
	backdrop/collector/ga.py
	tests/backdrop/collector/test_ga.py
Code style fix

@maxfliri