This allows the signing key to be read directly from S3 using a path like
/s3/<bucket>/<path/to/signing.key> or /gcs/<bucket>/<path/to/signing.key>.

This switch statement doesn't do what I thought it does

Switch from json to hcl configs

This is backward-compatible with the JSON config format - this is a
non-breaking change.

HCL treats config blocks as repeated fields so the config has to be
unmarshalled into a struct comprised of []Server, []Auth, []SSH first.

client: use a config file in addition to command-line flags

Run more CI checks

Re-enable go 1.5
Run `go build`, `gofmt` and `go vet` in addition to tests.

To allow for easier development on localhost where one cannot get a
root-CA signed TLS certificate, add a new validate_tls_certificate
option to the configuration file which optionally allows for certificate
chain checking to be disabled.

Set expiry time in the github auth package

Add config.json to a gitignore to prevent it being versioned.

Don't allow wide-open Google or Github configs

Fail loudly if either the google_opts domain value or github_opts organization
values are not set in the configuration. The lack of these values means that
 a) in the Google case any @gmail.com address will be allowed
 b) the Github case any Github user will be allowed.

This was previously documented but left as a foot-gun in the code.

Future commits will allow for explicit wildcards to be set.

Also add a Dockerfile