add draft support to mongodb, require pymongo

[cocoon-project]

a first shot to add mongodb as a backend
needs pymongo ( add to requirements.txt ?)

[escalant3]

Hi @cocoon-project,

It looks good!

A couple of things:

• The if __name__ part could use a clean up. There's a trailing print and some commented out lines that could be removed.
• Feel free to add pymongo to requirements.txt. It's how the other supported platforms work. It'd be interesting to have optional requirements depending on the system that it's going to be used in the future.
• You could update the README with some usage examples if you want.

Thanks for your contribution!!

[cocoon-project]

I work on several other backends , redis , and the native python shelve library
and a way to import dependencies as needed.
i will propose it as soon as tested