Read protobuf version directly in setup.py

[tilsche]

fixes #162

This will need more testing. Maybe we need to use out_dir instead of the current working dir to find the module file. Nothing is documented, it all depends on the installation in which setup.py is invoked, local install, packaging, pip, or whatever...

[tilsche]

Tested today

• [x] run pip install on a directory with existing protobuf files with protoc installed -> OK, detect version from module, no files generated
• [x] run pip install on a directory without existing files with protoc installed -> OK, files generated
• [x] run pip wheel on a directory with existing files -> OK, wheel generated with protobuf files
• [x] run pip wheel on a directory without existing files -> OK, wheel generated with new protobuf files
• [x] run pip install on a built wheel on a system without protoc installed -> OK
• [x] run pip install on a directory with existing protobuf files without protoc installed -> OK
• [x] run pip install on a directory without existing files without protoc installed -> SHOULD FAIL