Implement multiple resource types in Packer

[lookeypl]

Continuation of #1

Right now Packer handles only one type of resource - files from filesystem. Following improvements must be done to Packer code to handle other resources:

• [ ] PackerElement must be turned to abstract class and derived classes PackerElementFile and PackerElementBuffer must be implemented.
  • [ ] PackerWriter should hold PackerElement array.
  • [ ] WritePAK should call virtual methods from PackerElement to write files.
• [ ] AddFile verison for Buffer must be implemented (there is already a stub in code).
• [ ] Tests for Buffer resource must be written.