Snowhouse v3.1.0

This is a minor update of Snowhouse.

The only new feature is that you can use Snowhouse as a CMake library, see #38 by @HauserV. For this, the library is moved into an include subdirectory.
Note that this is a compatibility-breaking change if you use the master branch as a submodule, which, however, is not a documented use of Snowhouse. In the headers-only branch, nothing changes (except the documentation).