Add extension method MapPath to HostingEnvironment

[Praburaj]

Add extension method: MapPath

Moving a part of #157 into a separate item.

[tyson-benson]

Hi @Praburaj Can I ask what kind of timeframe we've got for this extension? I got stuck on this when attempting to port an existing project to ASP.NET 5

[davidfowl]

It's literally an extension method that does Path.Combine 😄

[tyson-benson]

That gave me a good chuckle, i figured it would be more complicated. Would it go something like this?

public static string MapPath(this IHostingEnvironment env, string relativePath)
{
    return System.IO.Path.Combine(env.WebRoot, relativePath);
}

[Praburaj]

@tyson-benson @davidfowl I sent out a PR. I'll add some tests before checkin.