Enable test coverage analysis using ass (fix #4)

This script comes from fxa-auth-server but it is always run with
the test in this project.
Add more email_record unit tests to get 100% coverage
Merge pull request #11 from fmarier/ass_coverage

Ass coverage (fix #4)
Make the InvalidAgentInterval configurable
Pass the missing BLOCK_INTERVAL_MS to the ip_record
Check for blocks against the right interval
Make the now() function configurable to facilitate testing
Copy the retryAfter function from email_record.js
Add unit tests for ip_record.js