Switch from /bin/sh to /bin/bash

You have bashisms in your script and /bin/sh might not always be bash. In that case the script would fail (for example on Ubuntu).
Remove unneeded shebang in nvm.sh
Added command to get current nvm environment.
Added test for `nvm current`
For zsh users

i use zsh and i did this update in install.sh local, is good for you too :)
Properly remove the tar file.
Fixed bash error caused by lack of double quotes
Add -f
Merge pull request #311 from loganfsmyth/remove-tar

Properly remove the tar file.
Merge pull request #312 from quiqueg/master

Fixed bash error caused by lack of double quotes
Merge pull request #305 from marioidival/patch-1

For zsh users
Addressing comments raised in #171
Universal test of unsetopt command

According to http://stackoverflow.com/a/7522866/189431 it should work in "bash, zsh, ksh and sh (as provided by dash)".
Merge pull request #322 from koenpunt/fix-unsetopt-test

Universal test of unsetopt command. Closes #300
Slurp test unsetopt stderr
Merge pull request #324 from hurrymaplelad/quiet_type

Slurp stderr from  test unsetopt
helper method for testing existence of commands
Merge pull request #326 from koenpunt/has-command-helper

helper method for testing existence of commands
fix copy-packages
fix copy-packages in zsh (without breaking sh and bash)