Solves [ReferenceError: process is not defined] when using with Jade

When using Jade and compiling a template for the first time, there's a lot of this in the log:

```
[ReferenceError: process is not defined]
```

It seems the process variable is not being made available to the vm context by UglifyJS, which is a dependency of several of Jade's dependencies.  Making it available then leads to:

```
[ReferenceError: Buffer is not defined]
```

This pull request makes both process and Buffer available to the created context in order to quieten the errors in the logs.

I hope you'll consider merging it.