Prepare for usage as package

[bjrne]

Like described in #14, in its current state, the repository cannot be imported due to dashes here and there. In addition to the small changes here, the repository name would need to be changed to contain underscores instead of dashes.

Since its easy to miss in the github view, this PR:

• [x] renames grobid-client.py to grobid_client.py
• [x] changes the import in grobid_client.py to include the package name
• [x] adds an empty init file
• [x] Renames the reposity to an underscore version (I cannot propose this)

EDIT: This seems to be related to #4, which would probably work after this PR.

[kermitt2]

This is covered with PR #17 which includes doc update, test and some more details, thanks a lot !