Add tasks for publishing to nuget

[timrwood]

RE: https://github.com/moment/moment/issues/1041 and https://github.com/moment/moment/issues/1816, it would be nice to include Nuget in the release process for moment-timezone.

There is a package in place already at https://www.nuget.org/packages/Moment.Timezone.js/, maintained by @corydeppen.

Moment core already publishes to nuget, so we can look at that implementation for reference. https://github.com/moment/moment/blob/develop/tasks/nuget.js

@corydeppen, are there any gotchas or advice to keep in mind when implementing this? Do you have an existing .nuspec file or a repo anywhere?

[corydeppen]

I think the only real hurdle is running the NuGet exe if you're not on Windows, but if you have grunt-nuget working for the Moment core project you should be fine if you're using that as a reference. I've included the latest nuspec file below.

<?xml version="1.0"?>
<package xmlns="http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd">
  <metadata>
    <id>Moment.Timezone.js</id>
    <version>0.5.3</version>
    <authors>Tim Wood</authors>
    <owners>Cory Deppen</owners>
    <description>Parse and display moments in any timezone.</description>
    <projectUrl>http://momentjs.com/timezone/</projectUrl>
    <iconUrl>http://pbs.twimg.com/profile_images/482670411402858496/Xrtdc94q_normal.png</iconUrl>
    <licenseUrl>https://raw.githubusercontent.com/moment/moment-timezone/develop/LICENSE</licenseUrl>
    <dependencies>
      <dependency id="Moment.js" version="2.6.0"/>
    </dependencies>
    <tags>JavaScript date time timezone browser node.js</tags>
    <releaseNotes>
      - Updated data to IANA TZDB 2016c.
    </releaseNotes>
  </metadata>
</package>