Merged installer and gitless installer into one

As a followup of #339, `install.sh` and `install-gitless.sh` combined into a single file (`install.sh`)

Install directory can be set using `NVM_DIR`, profile can be set using `PROFILE` and source can be set using `NVM_SOURCE`. Eg:

```sh
curl https://raw.github.com/creationix/nvm/master/install.sh | \
  NVM_DIR=/usr/local/nvm PROFILE=/etc/profile sh
```

Tested in zsh, bash, sh and ksh.