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
Detect if curl or wget is installed
Use 2-space indentation for consistency
Fix for running with bash, sh, and zsh
Merge pull request #370 from jpadilla/patch-1

Detect if curl or wget is installed to download NVM_SOURCE
Overwritting GREP_OPTIONS on local script

(recommiting to automerge into current master)
Merge pull request #310 from danieltdt/grep-options

Overwritting GREP_OPTIONS on local script
Merge pull request #270 from hachre/patch-1

Switch from /bin/sh to /bin/bash
Unset the executable bit on nvm.sh, since it's meant to be sourced
Adding a CONTRIBUTING.md file
nvm exec wrapper to execute commands with a specific node version
Use has util method and use comparison opts
Merge pull request #372 from koenpunt/patch-1

Use has util method and use comparison opts
Merge pull request #319 from koenpunt/nvm-exec

nvm exec wrapper to execute commands with a specific node version
Files should end in trailing newlines
fixed nvm_version result when multiple versions match the given pattern
added test for using the latest of two versions
Merge pull request #366 from messju/nvm_version_fix

fixed nvm_version result when multiple versions match the given pattern
Replaced echo for printf when using formatted strings
Updated version list, now also compatible with zsh