Add CSS format export of Inventories.

This is a fairly simple export of the `Inventory` class to the CSS format. It only covers the `affiliation`, `network`, `site`, and `sitechan` relations. The `sensor` and `instrument` relations seemed relevant, but are not implemented at this time, since I didn't need them.

The standard is a bit vague on the format of the `lddate` fields. I have picked something that is hopefully unambiguous (if UTC is assumed), while still fitting the space allotted.

Alas, I have no real test files. The tests given here just match the default result from `read_inventory`. They appear to follow the spec and are able to load in `mineos` (which is my target). If someone has Antelope or rdseed, it might be useful to cross-check against them.
> An additional possibility would be to return a dictionary with strings or file-like objects as the values. But I cannot think of a case where this would be needed and it is not supported by the rest of ObsPy so I think only supporting filenames is totally fine.

That sounds a bit overdone and more confusing than useful, to be honest.. ;)