Made ScreenCapture class public again.

Don't know the reasons of making them internal in previous touching commit, but some code already uses it, so making it internal is an unnecessary breaking change.

Author: ivan-danilov

src/TestStack.White/ScreenCapture.cs

@@ -12,7 +12,7 @@ namespace TestStack.White
     /// <summary>
     /// Provides functions to capture the entire screen, or a particular window, and save it to a file.
     /// </summary>
-    internal class ScreenCapture
+    public class ScreenCapture
     {
         // This code is a modified version of many similar classes along the same lines. There is no one source I can credit with this class