Add coverage reporting and uploading

Upload our coverage data to codecov.io and aggregate it on test runs.

codecov.yml

@@ -0,0 +1,43 @@
+codecov:
+  branch: master
+  bot: null
+
+coverage:
+  precision: 2
+  round: down
+  range: "60...100"
+
+  notify:
+    irc:
+      default:
+        server: chat.freenode.net
+        channel: '##python-code-quality'
+        branches: master
+        threshold: 2
+        message: null
+
+  status:
+    project:
+      default:
+        target: auto
+        threshold: null
+        branches: null
+
+    patch:
+      default:
+        target: auto
+        branches: null
+
+    changes:
+      default:
+        branches: null
+
+  ignore: null
+  fixes:
+    - .tox
+
+comment:
+  layout: "header, diff, changes, sunburst, uncovered, tree"
+  branches: null
+  behavior: default
+