Support multiple source files for Run()

Currently when multiple source files are passed to Run(), rizla creates multiple projects. When creating a project that uses platform-specific go files, it is necessary to pass in multiple .go files to `go run`: http://stackoverflow.com/questions/26920969/why-does-the-go-run-command-fail-to-find-a-second-file-in-the-main-package

It would be nice to have a Project handle multiple main source files to support this.

In support of: https://github.com/kataras/iris/issues/421