Recover from panics in ServeHTTP
Log panics to JSON error log.

First pass at this.  Writing to a hardcoded logfile in /tmp for now.
Configure error log file from Env variable.

Defaults to writing to STDERR
Log errors triggered in the BackendTransport.

This will catch HTTP errors generated due to failure to talk to a backend.
Add more details to logged fields.
Refactor creation of Loggers
Write entries to logfile in a separate goroutine

This will avoid blocking the request serving thread if there are lots of writes going on.
Allow router debugging from tests by ENV var
Merge pull request #40 from alphagov/error_logging

Log generated HTTP errors in JSON format

[Delivers #58081650]