Add appveyor config

appveyor.yml

+build: false
+
+environment:
+  matrix:
+    - PYTHON: "C:/Python36-x64"
+
+install:
+  - "SET PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
+
+  # Upgrade to the latest version of pip to avoid it displaying warnings
+  # about it being out of date.
+  - "pip install --disable-pip-version-check --user --upgrade pip"
+
+  # Create poetry virtualenv
+  - pip install poetry --pre
+
+  # Install dependencies
+  - poetry install -v
+
+test_script:
+  - "pytest --cov=poetry --cov-config=.coveragerc tests/"