Add codecov.io coverage tracking

* Every Linux build tracks coverage when running Debug mode
* OS X not supported yet (Future WIP)
* Our own unit tests, non-default reporters and Clara are ignored

codecov.yml

@@ -0,0 +1,14 @@
+codecov:
+  branch: master
+  ci:
+    # Don't wait for AppVeyor build to finalize coverage information
+    # We don't have a way to generate coverage from it anyway
+    - !appveyor
+
+coverage:
+  ignore:
+    - "projects/SelfTest"
+    - "**/catch_reporter_tap.hpp"
+    - "**/catch_reporter_automake.hpp"
+    - "**/catch_reporter_teamcity.hpp"
+    - "**/external/clara.hpp"