Move main package to top-level

Jussi Virtanen pointed out that the installation one-liner looks like a
typo because of the "capstan/capstan" postfix:

  go get github.com/cloudius-systems/capstan/capstan

Fix that by moving main package to top level.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #60 from penberg/main-package

Move main package to top-level