Use full path to nsenter in docker-enter

* Fixes "nsenter: not found" error
Merge pull request #12 from md5/nsenter-not-found

Use full path to nsenter in docker-enter
docker-enter() function simplified

there is no need to install docker-enter script via curl as the installer already does it:
https://github.com/sequenceiq/nsenter/blob/master/installer#L6
Merge pull request #13 from sequenceiq/docker-enter-function-on-boot2docker

docker-enter() function simplified
Merge pull request #1 from jpetazzo/master

Update from original