Single file compile by `grunt-contrig-watch`

Added single file compile wicth grunt-contib-watch.

But you have to turn of the spawn option of the watch module.

E.g:

{
  watch: {
    options: {
      spawn: false
    },
    soy: {
      files: ["_src/tmpls/*.soy"],
      tasks: ["soycompile:main"]
    }
  }
}

**Alternativly you can use the grunt-contrib-watch replacement grunt-regarde witch works since version 0.1.0 of grunt-soy-compile