Fixed an issue in which full paths wouldn't work properly

On Content.cs (I'm guessing by mistake) there was an addition of 2 strings that form a path. However, when the path provided is a full path then it doesn't make sens cause you end up with a full path followed by another full path. Using Path.Combine the standard library this case is taken care of.
```c#
rootPath + item.Path -> Path.Combine(rootPath, item.Path)
```
As for the other change in NPLConfigReader.cs, if the path provided is a full path instead of a relative path, then we don't really want to remove anything as it is already a correct path. So If the path is rooted just use the path as is:
```c#
string name = file;
if (!Path.IsPathRooted(file))
     name = file.Remove(0, rootDir.Length).Replace('\\', '/');
var newItem = new Item(name);
```

NoPipeline/NoPipeline/Content.cs

@@ -110,8 +110,8 @@ public void CheckIntegrity(string rootPath)
 				// Don't include if the file doesn't exist.
 				if (File.Exists(Path.Combine(rootPath, item.Path)))
 				{
-
-					DateTime itemLastModified = File.GetLastWriteTime(rootPath + item.Path);
+					// Using Path.Combine to prevent issues when one of the paths is a full path already
+					DateTime itemLastModified = File.GetLastWriteTime(Path.Combine(rootPath, item.Path));
 
 					var relativeItemPath = Path.Combine(rootPath, Path.GetDirectoryName(item.Path));
 

NoPipeline/NoPipeline/NPLConfigReader.cs

@@ -76,7 +76,10 @@ public void Read(Content content, string configPath)
 
 				foreach (var file in files)
 				{
-					var name = file.Remove(0, rootDir.Length).Replace('\\', '/');
+					// If the fail is already a valid and full path, we don't want to modify it.
+					string name = file;
+					if (!Path.IsPathRooted(file))
+						name = file.Remove(0, rootDir.Length).Replace('\\', '/');
 					var newItem = new Item(name);
 
 					Console.WriteLine("    Reading " + name);