Add "pip install" support

[bohemianoid]

This adds support to install icsv2ledger with pip:

$ pip install https://github.com/quentinsf/icsv2ledger/archive/master.zip

pip makes icsv2ledger available system wide:

$ icsv2ledger [options] -a STR [infile [outfile]]

I hope, this is a useful addition to icsv2ledger. Looking forward to your feedback.

[peterdc]

Thank you for this. I had set this up myself with a previous version, but a more recent commit broke it for me. This pull request resolves those issues.