chore(nuget): add .nuspec

So here's the `.nuspec` I talked about. I've already pushed the package to [nuget.org](https://www.nuget.org/packages/angular-strap), I'm only keeping it here for future releases. Also, should I add a dependency on [angularjs.animate](https://www.nuget.org/packages/AngularJS.Animate)?

Fixes #355