Get nuget working behind a proxy.

Here's what I did to get this working with my corporate proxy that uses NTLM
authentication. I downloaded NuGet.exe from a link on this page and then ran the
following commands (which I found in the comments to this discussion on
CodePlex):

    nuget.exe config -set http_proxy=http://my.proxy.address:port
    nuget.exe config -set http_proxy.user=mydomain\myUserName
    nuget.exe config -set http_proxy.password=mySuperSecretPassword

This put the following in my NuGet.config located at %appdata%\NuGet (which maps
to `C:\Users\myUserName\AppData\Roaming` on my Windows 7 machine):

    <configuration>
        <!-- stuff -->
        <config>
            <add key="http_proxy" value="http://my.proxy.address:port" />
            <add key="http_proxy.user" value="mydomain\myUserName" />
            <add key="http_proxy.password" value="base64encodedHopefullyEncryptedPassword" />
        </config>
        <!-- stuff -->
    </configuration>

Incidentally, this also fixed my issue with NuGet only working the first time I
hit the package source in Visual Studio.