Added option to set roots public SSH key

A new option called rootssh may contain the public SSH key for the root
user, which will be placed in /root/.ssh/authorized_keys, so no
password is needed to login using SSH and the root user.
Providing this option also prevents changes to SSH config on jessie, so
root login using password will stay disabled on that distribution (as
it should be).
Added options to create a system user

Several new options have been added (create_user, userpw, userssh,
user_is_admin) that allow the script to create a new user on the
installed system. A password is set for the new user, optionally a
public SSH key is added to authorised_keys, and if user_is_admin is set
to yes/true, the user will be able to use sudo (which is added to the
installation) for root tasks.
Added option to disable root password login

A new option called disable_root may be added, and if set to Yes/True
it will prevent any password login for the root user. A password will
*not* be set for the root user, so make sure a system user with sudo
access is created (using create_user and user_is_admin)
Cleaned up echoes, added delay before install

Added spaced around the ‘=‘ sign while displaying setup and added a
delay after displaying it, so the user has some time (5 seconds) to
review it and possibly power off the device
(maybe an option to cancel and drop to shell might be in order here…)
Fixed some displayed text
Fixed chown issue for user ssh dir
Fixed another typo *blush*
Fixed another mistake

forgot the reason for removing the /rootfs from the variable in the
first place… silly mistake *sigh*
Added groups from 'official' raspbian

when a user account is created, it’s added to a number of groups, these
are now listed in $user_groups. if the account is to be an admin
account, additional groups from $admin_groups are added to the list
Removed a stale comment
Removed some groups that don't exist by default

There are some groups that exist in the ‘official’ raspbian
distribution, but not the custom built raspbian.
Since these groups were referenced in the install script, user creation
failed. This fixes it
Changed create_user to username
Changed rootssh to root_ssh_pubkey
Changed userssh to user_ssh_pubkey
Changed README.md
Added explanation for SSH key to README.md
yeah... typo *blush*
Merge pull request #97 from goranche/master

Changed creation of a temporary file, it now prefers mktemp, but will use tempfile if used. if all else fails, it creates a temp file in /tmp
Merge branch 'master' into v1.0.x
Merge pull request #99 from goranche/v1.0.x

merged changes from master