Introduce copy METHOD for installation

This method copies the folder containing install.sh (normally also containing the repo) to  - unless of course the install.sh being executed is in  (where nvm was installed)

This is useful for vagrant to test local changes instead of pulling them from git and thus testing what's on the server and not local changes

As [requested](https://github.com/creationix/nvm/pull/578#discussion_r20406227), here is a seperate PR for the copy method for installation.
Can you elaborate a bit on the problems that would be caused by getting the location of the linked script versus the non-linked path? It seems like the presence of the link shouldn't affect the copy - ie, it would still work with the linked path.

A comment with a link to the SO answer would indeed be helpful. Using a function makes it very clear that the sole purpose is to output a directory path and that you're not intending to rely on other side effects :-)