Better combo curl/wget handling

Fixes #451
The unsupported arguments are `--progress-bar`, which is not required so we'll just strip that, and `-L` (follow location) is the default in wget so we can strip that as well, and `-C` (continue transfer) has an equal in wget: `-c`:

```sh
nvm_curl() {
    ARGS="$* "
    ARGS=${ARGS/--progress-bar /}
    ARGS=${ARGS/-L /}
    ARGS=${ARGS/-s /-q }
    ARGS=${ARGS/-o /-O }
    ARGS=${ARGS/-C /-c }
    wget "$ARGS"
}
```

And then there is `-I`, but we can replace that for a version comparison, as it seems like that since version 0.5.1 they all end up in a subdirectory on http://nodejs.org/dist anyway:
```sh
[ $(nvm_normalize_version "$VERSION") -ge $(nvm_normalize_version "0.5.1") ]
```