Export DOCKER_HOST environment variable

Regression from 1712714cb8c77344d24db594daf38df51bf0dd82
Merge pull request #41 from michaelneale/master

will only run apt-get update when docker version is bumped
Merge pull request #40 from noplay/export-docker-host-envvar

Export DOCKER_HOST environment variable
Remove redundant start instruction

This is explained below.
Specify specific stable version to install
Fix typo in help docs

"Environment" was misspelled.
Fix typo in README

"Environment" was misspelled.
Merge pull request #42 from cschneid/patch-1

Fix typo in help docs
Merge pull request #43 from cschneid/patch-2

Fix typo in README
Improve shell environment explanation
docker version to be downloaded is now correct, and updated to 0.8