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.
Second part of rename.
Fix missing abstract keyword.
Fix reference to renamed PvrtcBitmapContent class.
Merge pull request #1688 from slygamer/dxtbitmapcontent-packedvector

DxtBitmapContent and PackedVector additions