Make shasum the lowest priority checksum command.

This fixes false "Checksums do not match" errors on systems where shasum
is a symlink to sha256sum or otherwise.
* Make the creation of a symlink an overridable default behaviour
* Unit tests
* We print stats only on failure
* || true
Merge pull request #514 from stelcheck/master

Make the creation of a symlink an overridable default behaviour
Add unit tests for nvm_checksum.
Run cleanup on die in nvm_checksum unit test.
Merge pull request #512 from kevinwang/shasum-lowest-priority

Make shasum the lowest priority checksum command.
Don't try to copy `npm` itself with `nvm copy-packages`.

Fixes #504.
Properly pass a space-separated list of packages to `npm install`

Fixes #490.
Remove unused $ROOT variable.
Quote the version.
`npm ls` has a bug in some versions when "--parseable --depth=0" is used together.