Create parent directory to contain all generated brickstrap files

[dlech]

Currently, brickstrap creates files and a rootfs directory in the current directory. I think it would be better if we contained all of the generated files in a parent directory. So, we have...

<dir>/                        created by create-conf
  + rootfs/                   created by create-rootfs
  + brickstrap.conf           created by create-conf
  + multistrap.conf           created by create-conf
  + rootfs.tar                created by create-tar
  + <dir>.img                 created by create-image
  + <dir>-report.txt          created by create-report

[dlech]

Fixed by #28