Added native support for Windows.

This change adds a native Windows port (port_windows.h) and a
Windows Env (WindowsEnv).

Note1: "small" is defined when including <Windows.h> so some
parameters were renamed to avoid conflict.

Note2: leveldb::Env defines the method: "DeleteFile" which is
also a constant defined when including <Windows.h>. The solution
was to ensure this macro is defined in env.h which forces
the function, when compiled, to be either DeleteFileA or
DeleteFileW when building for MBCS or UNICODE respectively.

This resolves #519 on GitHub.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=236364778

.appveyor.yml

@@ -0,0 +1,38 @@
+# Build matrix / environment variables are explained on:
+# https://www.appveyor.com/docs/appveyor-yml/
+# This file can be validated on: https://ci.appveyor.com/tools/validate-yaml
+
+version: "{build}"
+
+environment:
+  matrix:
+    # AppVeyor currently has no custom job name feature.
+    # http://help.appveyor.com/discussions/questions/1623-can-i-provide-a-friendly-name-for-jobs
+    - JOB: Visual Studio 2017
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      CMAKE_GENERATOR: Visual Studio 15 2017
+
+platform:
+  - x86
+  - x64
+
+configuration:
+  - RelWithDebInfo
+  - Debug
+
+build:
+  verbosity: minimal
+
+build_script:
+  - git submodule update --init --recursive
+  - mkdir build
+  - cd build
+  - if "%platform%"=="x64" set CMAKE_GENERATOR=%CMAKE_GENERATOR% Win64
+  - cmake --version
+  - cmake .. -G "%CMAKE_GENERATOR%"
+      -DCMAKE_CONFIGURATION_TYPES="%CONFIGURATION%"
+  - cmake --build . --config "%CONFIGURATION%"
+  - cd ..
+
+test_script:
+  - cd build ; ctest --verbose ; cd ..