Fixed/updated the GDI+ backend guard classes to conform to the current design (added the appropriate typedef to the gp_image class, moved the gp_guard class to the gp_extern_lib_guard.hpp header, cleaned up the said header...).
Updated the LibTIFF and GDI+ backends to conform to the current design (moved the X_image classes to the public/gil namespace, renamed the LibTIFF backend header appropriately...).
Added various new utility/guard classes to the libx_shared.hpp and windows_shared.hpp headers.
Moved the 'memory_chunk' typedefs to the shared.hpp header.
Moved the formatted_image_traits<> template to the public/gil namespace, updated all backends accordingly.
Decided on first global design/guidelines for write functionality (added the the formatted_image<>::writer_for<> template class along with various helper wrapper classes). Added write capabilities (writer implementations) to all backends.
Added the utility read() and write() formatted_image<> static member funcitons.
Added the view_data_t typedef to the formatted_image<> template (saved some verbosity in the concrete classes).
Minor other refactoring, optimization and stylistic changes.
