Supports a globally-installed nvm with user installs

This change allows a globally-installed NVM to be used by non-root users. It sets NVM_PATH to install to ~/.nvm, while NVM itself is installed to /usr/local/src/nvm.
The cookbook's LWRP to install node versions will still install to the global /usr/local/src directory. For a regular user to use the system installed packages, they must export NVM_DIR=/usr/local/src/nvm and source /usr/local/src/nvm/nvm.sh again, or explicitly update PATH=/usr/local/src/nvm/[v0.10.24]/bin and NODE_PATH=/usr/local/src/nvm/[v0.10.24]/lib/node_modules