Added support for multiple NuGet repositories.

I added support for multiple NuGet repositories defined in PackageConfig.xml. So it now can looks like this:

<?xml version="1.0" encoding="utf-8"?>
<PackageConfig>
  <RepositoryUrl>https://packages.nuget.org/api/v2</RepositoryUrl>
  <RepositoryUrl>D:\LocalNuGet</RepositoryUrl>
  <Packages />
</PackageConfig>

This is handy when you are developing some plugins and you want to keep them internal or just test them from internal repository before submitting them to nuget.org.

The save and load methods were changed only little bit here it looks like I change them whole. I guess it is because some difference between spaces/tabs in my Visual Studio.

I changed the format in which the urls are stored. Original url was directly converted to Uri but also keep in the string. I am saving urls only as a strings and do same conversion right before the repository is created. I am not sure if the public property RepositoryUrl was already used somewhere but when I changed it to RepositoryUrls everything seems to be fine.