Revise `egoio.db.connection()`

[gplssm]

The entire function connection() is a bit clumpsy and hampers easy integration/extension of its functionality. I would suggest to make a minor release (i.e. v0.5.0) which breaks the convention of this function and provides a cleaned up interface for

• config file
• manual config by parameters
• easy passwordless

And, I would would suggest to rename the function to engine(), since it returns an engine.
Maybe this allows to retain the old function connection() for some time to warn about deprecation and remove it later.