Write fingerprints.json file.

Example (reformatted using JSONLint):

[
    {
        "nickname": "gabelmoo",
        "fingerprint": "F2044413DAC2E02E3D6BCF4735A19BCA1DE97281"
    },
    {
        "nickname": "moria1",
        "fingerprint": "9695DFC35FFEB861329B9F1AB04C46397020CE31"
    },
    {
        "nickname": "ndnop2",
        "fingerprint": "DE6397A047ABE5F78B4C87AF725047831B221AAB"
    }
]