Implement rudimentary seccomp support.
Merge pull request #32 from amluto/seccomp

Implement rudimentary seccomp support.
Add 'minibox' sandbox tool for use when running things like zip/unzip on user data.
Use 'minibox' to sandbox zip/unzip for backup/restore.

This also allows us to avoid the copies we were doing before, which lets us drop the fs-extra package. I already had a recursive-rmdir implementation. It's synchronous, but that seems OK for now.
After deleting a grain, return to home screen. Fixes #31.
Merge branch 'master' of github.com:sandstorm-io/sandstorm
Add some more seccomp filters, particularly to disable user namespaces because they are kind of scary.

Yes, Sandstorm itself totally relies on user namespaces, but we don't want sandboxed apps to do so.

Also re-enable ptrace in dev mode, because it's really useful. But keep in blocked when not running via spk dev.
Add comments to installer script describing exactly what it does.
typo
Set correct content type on backups and correctly quote the filename. This makes things work better on Firefox. Unknown at this point whether this will fix the issue in production where the .zip extension gets lost (issue #34).
Increase xz compression level for releases.
Add Brian and David to front page.
Fix home page rendering on iPad 1. Add demo link.
Tweak demo UX.
Update es6-promise to 1.0.0. `cast` has been replaced by `resolve`
Update capnp to v0.1.3
Add bridgeConfig to pkgDef and persist it through to grains.

Change http-bridge to read permissions list from this new config, and
pass it through getViewInfo. Also, pass allowed permissions to the
client webapp through X-Sandstorm-Permissions
Add userId to userInfo, generated from the account service ids
Add proper permission handling to http-bridge
Handle Demo/Anonymous users better when finding usernames