Build invalidation batch from paths in configuration

[rgabo]

Instead of having to include a valid invalidation batch, why not build the invalidation batch from a much simpler configuration?

Please see sspinc/grunt-cloudfront@30db42bd0290040392ea24ddd16da8adc82172d4 in our fork. This is the only difference and we would love to contribute this back in a backwards-compatible manner by checking the configuration whether it already includes Paths and CallerReference or it expects grunt-cloudfront to assemble it).

It would make it much easier to invalidate paths, especially if they are determined dynamically.