use `useradd` instead of `adduser` for install

as `adduser` is afaik either an alias for `useradd`, or sometimes a wild
wrapper around it, or even non-existant (i.e. archlinux), switch to the
more portable direct `useradd` call.

omitting `--group`, as `useradd(8)` states:

```
By default, a group will also be created for the new user (see [...])
```
Merge pull request #104 from zined/use_useradd_in_installation

use `useradd` instead of `adduser` for install
add systemd compatibility to install script
use proper systemd service type

as the sandstorm binary forks, reflect that behaviour in the systemd
unit configuration.
Merge pull request #106 from zined/add_systemd_compat_to_install

add systemd compatibility to install script
Update readme to reflect that Sandstorm can run in Docker.
Fix installation failure on some systemd distros.

Likely for backward compatability reasons, /etc/init.d exists in at least some
modern systemd distributions resulting in the installer failing attempting to
run update-rc.d, which does not exist. (CentOS 7, Fedora 20)

Instead, flip the logic so we look for systemd first, and fallback on init.d.
Fix minor issue with stopping existing systemd service during install.

Syntax for systemctl list-unit-files appears to no longer be correct,
specifying a service causes an error: "Invalid number of arguments."
Merge pull request #74 from jparyani/testing

Add integration tests
Merge pull request #110 from dgoodwin/install-systemd

A couple small fixes for installation on systemd distros.
Improve systemd detection in installer script.
get rid of old port-number-based urls in example nginx config
delete obsolete README tips
you need a wildcard DNS in any case
Merge pull request #112 from dwrensha/nginx

get rid of old port-number-based urls in example nginx config
Minor fix in usage docs for mapping directory for Dockerfile
Fix uploading spks for tests
Fix clearPackages to also delete grains
Output sandstorm.log on failed tests