Add appveyor build status
.. image:: https://travis-ci.org/lmcinnes/umap.svg
:target: https://travis-ci.org/lmcinnes/umap
:alt: Travis Build Status
+.. image:: https://ci.appveyor.com/api/projects/status/github/lmcinnes/umap?branch=master&svg=true
+ :target: https://ci.appveyor.com/project/lmcinnes/umap
+ :alt: AppVeyor Build Status
.. image:: https://coveralls.io/repos/github/lmcinnes/umap/badge.svg
:target: https://coveralls.io/github/lmcinnes/umap
:alt: Test Coverage Status
