TitleContainer Fixes

This fixes TitleContainer.OpenStream() to not allow absolute paths (as per-XNA) and in the process fixing #1295.

 - TitleContainer.OpenStream now throws an exception when passed an absolute path.
 - Added internal for TitleContainer.Location.
 - Removed most uses of ContentManager.RootDirectoryFullPath.
 - Unified the 5 copies of TitleContainer.cs into a single one for all platforms.

I tested this on WindowsGL and Windows8.  It could use some testing on iOS, Android, and MacOS.