Install function for testing

Following discussions about testing the install script and @ljharb's will to have many small pull requests, here's the first one :)

What it does:

* wraps the code of the `install.sh` that was actually performing the install into a function
* The function is executed at install but __not__ during tests
* First test for the install script (_is the function defined?_)


this whole function could be one line - `unset -f nvm_do_install nvm_has nvm_download install_nvm_as_script install_nvm_from_git nvm_reset`?