Modular command line interface

This is my take on what a command line interface could look like. It is not completely ready, but reasonably complete in order to start discussing pros and cons. Hence this PR is not meant to be merged just yet, but rather an RFC.

In particular, I'd be interested in thoughts on:

- How to test the beast? Currently this is done in the examples, which are aiming to become testkraut tests. There is also a dedicated 'pytest' command to aid this effort.

- Improvements of uniformity and structure of the commands?

- What other basic commands are missing? (I aim to replace the crippled 'dsinfo' with something saner -- maybe 'plot')

- Which parts of the old PyMVPA code can be made obsolete? I am looking at mvpa2.misc.cmdline, at least

- Ideas on how to improve re-use of PyMVPA modules and functionality through the cmdline interface? It should be pretty flexible already (see crossval --learner for an example), but there could be many ways to make it more convenient and avoid the need for scripts, without adding clutter -- one example is crossval --partitioner.

Let's talk.

PS: A merge around the end of the year would be a good timing in order to have a sufficiently large test window before the upcoming release that needs to be ready for the workshop on Mar 3.

PPS: the hyperalign command is a demo on how to re-use an algorithm class for the cmdline interface. However,
due to some unfinished work, this is currently broken. I am working on it right now, so better leave it alone for now