Allow to pass the client directory in bin/omero import

As it stands, the client_dir is constructed from the Context directory. This
commit allows to pass a custom client directory to the CLI import plugin.
Refactor test_import.py to use cli.invoke()

- Import and register the ImportControl class (requires __import__)
- Call cli.invoke() instead of using the library import_image
- Read the image/plate id directly from the stderr using capfd
Fix clientdir argument validator
Merge pull request #31 from sbesson/import-debug

Add client_dir argument and refactor test_import.py