Support combined moment+languages form of the library

As I mentioned in #940, I think there's a case to be made for bundling the main moment.js, and whatever languages a given user needs for their application, together, into just one file.  This simplifies loading of the library and the desired language immensely (no need to load moment and the language bundle separately, no need to define multiple AMD paths for each language).

So, in this series of commits, I introduced the ```grunt concatwithlang``` task (defined in tasks/concat-with-lang.js, modeled after tasks/concat-lang.js).  Run it without specifying ```--useLang``` on the command line, and it produces moment_and_alllangs.js, as seen in compiled, non-minified form in this pull request.  Run it with specifying, for example, ```--useLang=uk,cs``` and the file would instead only bundle those two languages.  This is a nice file size improvement for those that only need a limited subset of languages in their application (mine calls for 10-15, more or less), at least when compared to always loading the core moment.js and langs.js.