Support json content type

[danielemery]

Currently the main keys endpoint returns keys in a format that is suitable for use in an authorized_keys file without any parsing or modification.
This is a key features of the keys project and should be kept as the default behaviour.

However if the caller requests content-type: json they should instead receive the keys is json format. This is far more convenient to use programatically (eg by the cli in #24) and can provide all the metadata associated with they keys (eg tags).