Create check-in data structure

[dmp1ce]

This structure will be used to determine if the check-in is valid.

• date checked in
• fingerprint of user checking in
• Bitcoin block hash proof

Decide on format. For example: JSON, yaml, XML, Haskell serialization.

[dmp1ce]

This will be a cleartext signed message of a recent bitcoin block hash. Several public APIs will be used to check the time of a block hash.