Replace leveldb/ with vanilla 1.7.0
Remove Snappy support
Native Windows LevelDB port

Import native Windows LevelDB port by Hiram Chirino.

Extracted from from https://github.com/chirino/leveldb.git using
git diff dd0d562..aea83b7
Mingw support for Windows LevelDB port
Pre-Vista leveldb::port::InitOnce implementation
Update makefiles/gitian for LevelDB 1.7, Ubuntu Precise, MinGW-W64

Updates:
 * Update linux-mingw and bitcoin-qt.pro for LevelDB 1.7
 * Update gitian base VM image to precise
 * Use MinGW-w64 for Win32 builds
Use Boost 1.52 in Windows builds
Port leveldb to MinGW32

Several changes to make the native windows leveldb code compile
with mingw32 and run on 32-bit Windows:

Remove -std=c++0x dependency (modified code to use NULL instead of
nullptr)
Link with -lshlwapi
Only #define snprintf/etc if compiling with Visual Studio
Do not link against DbgHelp.lib (wrote a CreateDir instead of using
DbgHelp's MakeSureDirectoryPathExists
Replace %ll with %I64, because old versions of Windows' _vsnprintf do not
support %ll
Define WINVER=0x0500 so MinGW32 can use the 64-bit-filesystem Windows api calls

I also cleaned up makefile.mingw, assuming that dependencies would be in
the standard /usr/local/{include,lib} by default but allowing overriding
with make DEPSDIR=... etc
Build with -posix on MinGW