LevelDB 1.7 + windows updates (MinGW-w64, Boost 1.52, Precise gitian VM)

Several upgrades, each necessary to fix problem with the previous one. They could be separated, but at least this combination builds and works. It certainly needs a lot of testing, but I want to give it some exposure:
 * Upgrade to LevelDB 1.7
 * Switch to a native Windows LevelDB port (not anymore via Boost)
 * Switch to MinGW-w64 (32-bit mode) for Windows builds, as the new Windows port required a more recent GCC/environment than provided by MinGW32.
 * Switch to a 32-bit Precise image for Windows builds (which provides MinGW-w64 with GCC 4.6.3).
 * Switch to Boost 1.52, as Boost 1.50's thread library doesn't build on MinGW-w64.