Revamp CI integration

 - Add CircleCI for faster OS X builds
 - Use fewer workers on Travis for faster builds (3.4/3.5 dropped)
 - Move coverage uploads into custom Python for Python 2.6 support
 - Installtion of dependencies is now handled in new "deps" task
 - Improve AppVeyor and Travis CI configs for better granularity
diff --git a/codecov.json b/codecov.json
new file mode 100644
index 0000000..2bec947
--- /dev/null
+++ b/codecov.json
@@ -0,0 +1,4 @@
+{
+  "slug": "wbond/asn1crypto",
+  "token": "98876f5e-6517-4def-85ce-c6e508eee35a"
+}