Use phantomjs module from npm and make tests run on Windows

I opened #707 a while back, but seeing how nothing has changed in that regard I figured I'd just do it myself.

While I was at it, I noticed that the tests fail to run on Windows because the test server was not expecting Windows-style path separators. I fixed that as well.

I also separated build dependencies and test dependencies in package.json so that by default you only get the build dependencies, and can then install phantomjs and express for testing purposes with `npm install --dev`.

Right now the actual tests don't pass on Windows, though. I'm still looking into that.
```
zepto.html - Finished in 1.93 s. – 176 tests passed (753 assertions)
ZeptoAjaxTest#testAjaxLoad: Failed assertion. Expected %o to be == to %o. simple ajax load

 simple ajax load

unexpected global: jsonp6
unexpected global: jsonp9
ajax.html - Finished in 1.09 s. – 1 failures, 0 errors (155 assertions)
```