Update to properly handle MultiCompiler instances

All of the compiler setup and plugin operations were assuming a singular Compiler instance vs. MultiCompiler constructs. This changes the behavior to allow for iteration over multiple compilers where necessary.

Fixes #31
With this config multiple compilers all write to the same file (overwrite). You better need to concat every output file (best in `this.readFile`)

``` js
webpackOptions.output.path = "/" + idx + "/";
webpackOptions.output.publicPath = "http://" + hostname + ":" + port + "/" + idx + "/";
```

where `idx` is the compiler index