add the ability to specify which npm mirror to fetch resources from

@lloyd, @zaach or @seanmonstar - could I get a review? This allows the user to specify an alternate NPM registry by specifying the `NPM_CONFIG_REGISTRY` environment variable. Using this with `http://registry.npmjs.eu` makes `npm install` blazing fast for the Persona repo for those of us in Europe.

For example:

```
~rm -rf node_modules
~time npm install                                                                                                             

> browserid@1.0.0-b2 preinstall /Users/stomlinson/development/browserid
> node ./scripts/lockdown

NPM Lockdown is here to check your dependencies!  Never fear!
using registry: registry.npmjs.org

> browserid@1.0.0-b2 preinstall /Users/stomlinson/development/browserid
> node ./scripts/lockdown

npm http GET http://127.0.0.1:52635/JSONSelect/0.4.0
npm http GET http://127.0.0.1:52635/cef/0.3.3
...
npm install  25.71s user 12.11s system 81% cpu 46.677 total
```

vs.

```
~rm -rf node_modules
~time NPM_CONFIG_REGISTRY=http://registry.npmjs.eu/ npm install 

> browserid@1.0.0-b2 preinstall /Users/stomlinson/development/browserid
> node ./scripts/lockdown

NPM Lockdown is here to check your dependencies!  Never fear!
using registry: http://registry.npmjs.eu/

> browserid@1.0.0-b2 preinstall /Users/stomlinson/development/browserid
> node ./scripts/lockdown

npm http GET http://registry.npmjs.eu/bcrypt/0.7.7
npm http GET http://registry.npmjs.eu/compute-cluster/0.0.6
...
NPM_CONFIG_REGISTRY=http://registry.npmjs.eu/ npm install  27.57s user 12.17s system 144% cpu 27.443 total