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.
Assume we have this scenario. Internal company nvm version with customizations.

```shell
root@localhost:~> cd /opt
root@localhost:/opt> git clone git@github.com:LoveIsGrief/nvm.git nvm # or git clone https://github.com/creationix/nvm.git
root@localhost:/opt> ln -s /opt/nvm/install.sh /usr/local/bin/nvm_install
user@localhost:~> nvm_install # Without symlink deref = cp /usr/local/bin $NVM_DIR
user@localhost:~> nvm_install # With symlink deref = cp /opt/nvm/ $NVM_DIR
```

>It seems like the presence of the link shouldn't affect the copy - ie, it would still work with the linked path.

Hmm... do you mean something like this?
```shell
> # Folders:
> find -name install.sh -exec ls -l "{}" \;
lrwxrwxrwx 1 ubuntu ubuntu 21 nov.  23 12:02 ./old_nvm/install.sh -> ../new_nvm/install.sh
-rw-rw-r-- 1 ubuntu ubuntu 0 nov.  23 11:59 ./new_nvm/install.sh
> cd old_nvm && ./install.sh # Still installs new nvm
```

Is that your concern?