windows: Add Appveyor CI builds for msvc port.

Appveyor is like Travis, but for Windows builds. The appveyor.yml configuration
will build the msvc port in all configuration/platform conbinations,
and run the tests for each of those.

windows/.appveyor.yml

+environment:
+  # Python version used
+  MICROPY_CPYTHON3: c:/python34/python.exe
+
+init:
+  # Set build version number to commit to be travis-like
+- ps: Update-AppveyorBuild -Version $env:appveyor_repo_commit.substring(0,8)
+
+configuration:
+- Debug
+- Release
+
+platform:
+- x86
+- x64
+
+build:
+  project: windows/micropython.vcxproj
+  verbosity: normal
+
+test_script:
+- cmd: >-
+    cd tests
+
+    %MICROPY_CPYTHON3% run-tests
+
+skip_tags: true
+
+deploy: off
+
+nuget:
+  disable_publish_on_pr: true