Added misc/tools/dbcopy.php as external dependency

`dbcopy` is an external tool that adds the following capabilities to volkszaehler:

  - incremental backup 
  - cross-database migration, e.g. from MySQL to SQlite

See `dbcopy.json` and https://github.com/andig/dbcopy  for configuration details.

@justinotherguy NEEDS TESTING