add a setup installation package to Flair

[samysung]

Pitch
Today, flair is used without package installation, just by using the python command line.
it would be great to have a real package installation with cli name entry points.

TODO

create and configure a setup.py file. Make src as a python package (add init.py file), and adds entry points for training routine (flair-train --config-file=/my/conf/file.yml) and for detection routine (flair-detect --conf=/my/conf/file.yaml see #2 ).