crawlers/github: Add function to check a repository structure.
crawlers/github: Add function to check a repository structure.

Some fields of a repository structure are mandatory to us, namely:

- ID
- Name
- Language
- CloneURL
- Owner (and Owner.Login)
- Fork

These fields are required either because their related column in the
database does not allow NULL values or because they are required for
further API calls. All other fields may be nil.

DevMine/crawld