Better OS X client instructions in README.md

The instructions on exporting DOCKER_HOST weren't quite right. It is probably better to just reference the official docs.
Merge pull request #37 from simonista/patch-2

Better OS X client instructions in README.md
Fix typo

Doh! :)
Merge pull request #38 from simonista/patch-3

Fix typo
Added support of Parallels provider
Merge pull request #41 from legal90/master

Added support of Parallels provider