Rename "capstan" package as "core"

The "go build" command won't work if there's a "capstan" directory in
the top-level directory. Rename the package to "core" instead.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #133 from penberg/core-package

Rename "capstan" package as "core"