Add note to README.md about DOCKER_HOST on OS X

Indicate that you can get the native OS X docker client to work by exporting DOCKER_HOST correctly.

Closes #12
Update README usage instruction to v0.5.4-1
add note about virtualbox version requirement

Issues #3 and #11 suggest that the scripts do not support old version of VirtualBox < 4.3. This version requirement should be noted up front in the readme to avoid others getting stuck on this issue (as I did just now).
Use boot2docker 0.8.0
Merge pull request #35 from jfoy/boot2docker_0.8.0

Use boot2docker 0.8.0
Merge pull request #32 from wojtekmach/patch-1

Update README usage instruction to v0.5.4-1
Merge pull request #29 from simonista/patch-1

Add note to README.md about DOCKER_HOST on OS X
Merge pull request #34 from wjordan/patch-1

add note about virtualbox version requirement
Use Vagrant Cloud based box
Update README