Webpack everything

This branch is an attempt to use webpack to build our tests instead of Browserify.

I'm currently having an issue that I hope @sokra can help with in this branch. When I run `./scripts/test` I get the following error:

```
ERROR [karma]: [TypeError: Not a string or buffer]
TypeError: Not a string or buffer
    at Hash.update (crypto.js:209:17)
    at sha1 (/Users/michael/Projects/react-router/node_modules/karma-webpack/index.js:129:7)
    at /Users/michael/Projects/react-router/node_modules/karma-webpack/index.js:151:15
    at MemoryFileSystem.(anonymous function) [as readFile] (/Users/michael/Projects/react-router/node_modules/webpack-dev-server/node_modules/webpack-dev-middleware/node_modules/memory-fs/lib/MemoryFileSystem.js:205:11)
    at doRead (/Users/michael/Projects/react-router/node_modules/karma-webpack/index.js:117:32)
    at Plugin.readFile (/Users/michael/Projects/react-router/node_modules/karma-webpack/index.js:120:3)
    at process._tickCallback (node.js:419:13)
```

Digging a bit deeper, it looks like the memory-fs module is having problems finding some of the files it needs. The two errors I'm seeing are:

```
/_js/modules/__tests__/Router-test.js Error: Path doesn't exists /_js/modules/__tests__/Router-test.js
/_js/modules/components/__tests__/DefaultRoute-test.js Error: Path doesn't exists /_js/modules/components/__tests__/DefaultRoute-test.js
```

@sokra Would you mind taking a look at this branch and telling me what I'm doing wrong?
I've always preferred the other way, have the `npm run` scripts simply call a `scripts/something` so that you don't do too much in here (and so the scripts are more portable), but anyway ... I don't care enough to say "no" to this commit.