Updated installers to be more consistent

As followup of #329, here a fragmented PR of the changes. Will rebase after review.

The diff is best viewed [without whitespace changes](https://github.com/creationix/nvm/pull/339/files?w=1) (appending ?w=1)

Original PR description:

-----
Updated installers, install directory can be set using `NVM_DIR`, profile can be set using `NVM_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 NVM_PROFILE=/etc/profile sh
```

Also, messages of both installers are now the same.

Tested in zsh, bash, sh and ksh.