Use node_modules lockdown

`node-gyp` kept failing for me because `process.env.HOME` was missing. This is fixed in `node-lockdown` in this commit [1302120a0b](https://github.com/mozilla/npm-lockdown/commit/1302120a0b9bcbd6344862413f6bf4c49d9ea686), however the changes aren't reflected in `browserid` because it uses `./scripts/lockdown`, which is a copy of `0.0.2`.

Instead, I changed the preinstall script to get lockdown and then run it from the `node_modules` folder.

If this is undesirable, I can just upgrade the version in `./scripts`