Avoid carrying over test status when no node version is installed

When sourcing nvm.sh, if there are no version of node installed the command
`nvm ls default` silentry fails, however it's return status code (`$?`) is
still set to non-zero, which means an error.

This patch simply avoids this issue by making sure the status code after
sourcing is ok so that people that put the returned status codes in their
command line prompt don't get an error every time `nvm.sh` is sourced.
remove continue from curl arg. nodejs.org doesnt seems to support continue, CDN effect perhaps?
Hide errors from `which unsetopt`

Errors occurred when it was run in bash.
changed npm url to use https (script has moved)
Merge pull request #144 from logosity/master

fix for npm install failure (corrects: Issue #91)
add /.nvm to NVM_DIR so we dont clutter up home directories
Revert "add /.nvm to NVM_DIR so we dont clutter up home directories"

This reverts commit a107b12d2ab3484f353e7f8f079a54fcf9ee3d8a.
add v to version if user does not specify it to simplify commands
updated readme
add ls-remote
install requested version automatically if it isnt available
made ls-remote actually usable
make install use a different nvm_version to make not specifying the v avoid clobbering aliases
add v back to ls-remote results
Merge pull request #145 from nathan-lafreniere/master

added ls-remote command
fixup automatically prepending the v for remote requests
Merge pull request #146 from nathan-lafreniere/master

fixup prepending of the v in ls-remote
make ls-remote work with no parameters in linux
Made string comparison also compatible for zsh
Merge pull request #149 from koenpunt/zsh-compat

Made string comparison also compatible for zsh