Multiple Repo support

[sh41]

• Ensure that each call to telemetry.attach/4 receives a unique handler_id based on the telemetry prefix of the repo.
• Ecto.DevLogger.install/2 now returns the result from telemetry.attach/4 allowing users to confirm successful setup.
• Add public Ecto.DevLogger.uninstall/1 to support calling telemetry.detach/1 with the correct handler_id.
• Add public Ecto.DevLogger.handler_id/1 to support introspection of telemetry handler ids.
• Add configuration to allow users to add the repo name into the log to enable distinguishing between queries on different repos.
• Refactor test setup & teardown to support setting up multiple repos.
• Tests for the above
• API Documentation

Fixes #10

[fuelen]

Hey @sh41

Thank you for the PR! Very good job, I like it!