Add an option to configure generator's output_dir

[vaind]

closes #312

• Check failed test: custom directory needs to exist.
• Imports in generated code are wrong (assumes generated code file is in top-most directory).
• Add documentation.
• Clean up integration test (currently just copy/paste from final test).

[greenrobot-team]

Thanks!
Changed to prefix imports with one or more ../ to fix imports in generated code file.
Fixed imports in test, reduced to minimal required.
Added documentation.
@vaind Maybe have a look at the import changes, can you think of a better solution?

[vaind]

Maybe have a look at the import changes, can you think of a better solution?

LGTM. I've made a minor change - not sure it's needed but doesn't hurt anything I guess