update nenv to rbenv 20140525

nenv is largely based on rbenv, but rbenv has moved quiiiiiite a lot since the original commit. this request "updates" a lot of the nenv code to match the current rbenv state, mostly by just applying
s/rbenv/nenv/
s/ruby/node/

This update brings
- help on subcommands is read from the Usage: comment, at the top
- shell completion is provided for bash and friends
- the shell subcommand, which switches node version based on presence of NENV_VERSION variable
- the hooks command, which allows plugins to define commands to run at various parts of a shim
- the root subcommand, which displays NENV_ROOT
- the --version subcommand, which calculates the version from the latest git tag + revision
- source copied to NENV_ROOT/sources rather than NENV_ROOT/source
- the list of remote releases was moved to the install -l command (the lack of which was the primary motivator for this update)