easier to load empty image into texture

Added a separate constructor to texture_data that allows for easy specification of an empty texture, e.g. for framebuffers. Also tweaked how unpack alignment is handled when loading image data, not specifying any alignment if the data pointer is NULL.
added framebuffer

Added the gladus::framebuffer class that wraps around OpenGL framebuffers. This is required for most of the modern OpenGL post-processing and effect techniques (shadow mapping, HDR, etc.).