Cleanup Gruntfile, streamline copy tasks

grunt watch:dev -> grunt watch
Cleanup / update Gruntfile

* Use proper grunt-umbraco-package task
* Use new structure for --target, eliminate deploy step by making `dist` configurable
* Move package files to /config/
* Cleanup watch/copy tasks, add atBegin: true
* Prevent "cleaning" non-local destinations (dangerous?)
* Cleanup full/base path config
* Whitespace, misc
Explain build details in README
Merge branch 'develop'