media/src/vector/SWC-logo.svg: Add Software Carpentry logo

media/src/vector/SWC-logo.svg: Add Software Carpentry logo

This logo is from the SWC assets repo [1], and it's content hasn't
changed since be1d25b (Adding source files for graphic identity,
2012-12-20).  It is distributed under the Creative Commons Attribution
3.0 license [2], although I'm not sure if Greg Wilson (the author of
be1d25b) is also the copyright holder who should be attributed or not.
Attributing "Software Carpentry" is probably fine.

[1]: https://github.com/swcarpentry/assets/blob/master/branding/logo/shirt-logo.svg
[2]: http://creativecommons.org/licenses/by/3.0/legalcode