Add /etc/os-release file to the installed system

Example:
```console
docker@boot2docker:~$ cat /etc/os-release
NAME=Boot2Docker
VERSION=1.1.2
ID=boot2docker
ID_LIKE=tcl
VERSION_ID=1.1.2
PRETTY_NAME="Boot2Docker 1.1.2 (TCL 5.3); master : 740106c - Tue Jul 29 03:57:24 UTC 2014"
ANSI_COLOR="1;34"
HOME_URL="http://boot2docker.io"
SUPPORT_URL="https://github.com/boot2docker/boot2docker"
BUG_REPORT_URL="https://github.com/boot2docker/boot2docker/issues"
```
Merge pull request #455 from tianon/os-release

Add /etc/os-release file to the installed system