These include configurations for both Win32 and x64 platforms. All project
settings have been normalised in property sheets (the ".props" files under
builds/msvc/properties) to simplify maintenance. Build artefacts are all
generated in platform-specific subfolders of bin, lib and obj directories.

Also enables the use of precompiled headers with MSVC10.

This significantly reduces the time required to compile libzmq with Visual
Studio on Windows. It should have no impact on other platforms.
Signed-off-by: Stuart Webster <sw_webster@hotmail.com>