Add an informative __main__.py (#620)

* Add an informative __main__.py

Give users an easy way to figure out what versions they're running.

* Why not more info!

* Add test

* No empty last line

* Make @alex happy

* DIAF Python 2.6

* Add cffi's version

* Make debug a module

* Add cryptography's compile-time OpenSSL
6 files changed