Allow custom outputs, not only riemann

[sallyruthstruik]

Thank you for this lib. But we use influxdb as our metric storage. So I've added functionality to write different outputs, and provided InfluxOutput class.

Also because in case of different outputs configuration may be complex, I've added support for .ini configuration.

[borntyping]

This changes a lot of things, it'd be very useful if you could split it up into multiple separate PRs - with_children in particular is quite notable as a breaking change on it's own. I do really like this though!

Some other changes I'd like to request:

• The list of changes should be added to the changelog in README.md, instead of creating a new file. It's also missing some things, like the previously mentioned with_children().
• Code style should match the existing code - i.e. import superman.x.y.z instead of from superman.x.y import z.
• It'd be useful (but not essential) to make riemann and influxdb optional dependencies.
• parser.get("supermann", "log_level", fallback="INFO") instead of try/catch in the config loading.
• Some general cleanup - methods like init_from_configparser are defined but not used.

[sallyruthstruik]

Hello, got it. Am I right, you want me to perform 2 pull requests, first with influxdb outputs and second with with_children recursive metric aggregation?

Other notes are clear for me.

[borntyping]

Yes please 😃