CRUD REST API [C++ + Crow + libpqxx + Postgres]

API should allow simple CRUD operations over artists entities.

Current status:

• List
• Retrieve
• Create
• Update
• Destroy

TODO:

• Run migrations automatically.
• Make PgRepo thread-safe (right now it is NOT).
• Bump libpqxx when jtv/libpqxx#476 will be merged.
• Compile & run in Docker.