Cannot render server-side

In node.js, when I call

```
require('flowchart.js')
```

I get the Raphael exception below:

```
/.../node_modules/raphael/dev/raphael.core.js:96
doc: document,
^

ReferenceError: document is not defined
at /.../node_modules/raphael/dev/raphael.core.js:96:18
at /.../node_modules/raphael/dev/raphael.core.js:15:26
at Object. (/.../node_modules/raphael/dev/raphael.core.js:19:2)
at Module._compile (module.js:413:34)
at Object.Module._extensions..js (module.js:422:10)
at Module.load (module.js:357:32)
at Function.Module._load (module.js:314:12)
at Module.require (module.js:367:17)
at require (internal/module.js:16:19)
at /.../node_modules/raphael/raphael.amd.js:16:23
```

Thank you for the good work !