Ambient Seismic Noise in ObsPy

This PR serves the purpose to discuss and implement what people want/expect from ObsPy in terms of handling ambient seismic noise data. I know a lot of people already have a bunch of code for this using ObsPy.

We would like to make the core of this functionality, mainly the processing and correlation handling, part of ObsPy. The goal is to have a stable and well tested toolbox for the basics of noise processing so people don't need to keep reimplementing those things.

I am not a noise person, so I don't know what exactly you require but the ideas right now are as follows:

* A set of preprocessing functions like 1-bit normalization, spectral whitening, ...
* A number of different correlation functions
* Objects to deal with CCs and CC stacks including serialization/deserialization

As a first step I propose to implement this in a `obspy.noise` submodule; we can always refactor this later on.

I'll contact a bunch of people whose github handles I don't know via email; please forward this to other people who might be interested as well.

@ThomasLecocq, @trichter, @lermert, @petrrr, ...

Please write your opinions and feel free to send PRs/directly push to this branch!

Cheers!