Add auto-build manifest for appveyor

Summary: Check in a simple auto-build manfiest so that developers can issue ad-hoc build for Windows in Appveyor.

Test Plan: Run it in test branch test_appveyor and it works well.

Reviewers: kradhakrishnan, rven, anthony, IslamAbdelRahman, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D43839

appveyor.yml

+version: 1.0.{build}
+before_build:
+- md %APPVEYOR_BUILD_FOLDER%\build
+- cd %APPVEYOR_BUILD_FOLDER%\build
+- cmake -G "Visual Studio 12 Win64" ..
+- cd ..
+build:
+  project: build\ALL_BUILD.vcxproj
+  verbosity: minimal
+test: off