DxtBitmapContent and PackedVector additions

Added bitmap content classes for DXT1,3,5.
DxtBitmapContent is now abstract.
Removed DxtBitmapContent.TryGetFormat (implemented by derived classes).
Added Bgr565, Bgra4444, Byte4 packed vector classes to all projects.
Corrected case for DxtBitmapContent, PvrtcBitmapContent.
DxtBitmapContent now matches XNA and C# naming conventions.
PvrtcBitmapContent follows suit.
ExternalReference relativeToContent now works properly.