Refactored NuGetPacker + Added NugetRestorer

1.Moved common nuget.exe functionality to NuGetResolver
2.Added NuGetRestorer - which adds extensions to restore soluton nuget dependencies
Example where solution is path to sln file:

	var target = Argument("target", "Build");
	var solution = Argument("solution", ".\src\test.sln");
	var configuration = Argument("configuration", "Release");
	var solutionDir = System.IO.Path.GetDirectoryName(solution);
	
	Task("Clean")
		.Does(() =>
	{
	    // Clean directories.
	    CleanDirectories(solutionDir + "/**/bin/" + configuration);
	    CleanDirectories(solutionDir + "/**/obj/" + configuration);
	});
	
	Task("Restore")
		.Does(() =>
	{
	    NuGetRestore(solution, 
	        settings=>settings.SetToolPath("C:/temp/priv/cakeTest/NuGet.exe")
	            .WithVerbosityDetailed());
	});
	
	Task("Build")
	    .IsDependentOn("Clean")
	    .IsDependentOn("Restore")
	    .Does(() =>
	{
	    // Build project using MSBuild.
	    MSBuild(solution, settings => 
	        settings.SetPlatformTarget(PlatformTarget.MSIL)
	            .UseToolVersion(MSBuildToolVersion.NET45)
	            .WithTarget("Build")
	            .SetConfiguration(configuration));         
	});
	RunTarget(target);