Create handlebars.js.nuspec for inclusion in NuGet

Added a .nuspec file for those people who use Visual Studio and NuGet and might want to use handlebars.js deployed from NuGet in their application.
Explicitly only include the handlebars.js from the dist folder
Merge pull request #505 from MikeMayer/master

Include a .nuspec package file to include handlebars.js on NuGet