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.
for consistency please do `local COPY` and `COPY=true` on two lines