Add coverage to travis and fix coverage collection (#38)

Fixes #26

Using pytest-cov (a pytest plugin) to measure the coverage of nbsmoke - which is itself a pytest plugin - is a little bit tricky. So just use coverage without the pytest-cov plugin.

.travis.yml

@@ -47,6 +47,11 @@ jobs:
     # like a python-using developer
     env: TOX_ENV=dev
 
+  - <<: *default
+    # measuring coverage of a pytest plugin using pytest-cov is slightly awkward,
+    # so we have a separate step for it
+    env: TOX_ENV=coverage
+
   - <<: *default
     stage: pip_package
     install: true

README.rst

@@ -6,6 +6,9 @@
     :target: https://ci.appveyor.com/project/pyviz/nbsmoke/branch/master
     :alt: See Build Status on AppVeyor
 
+.. image:: https://coveralls.io/repos/github/pyviz-dev/nbsmoke/badge.svg?branch=master
+    :target: https://coveralls.io/github/pyviz-dev/nbsmoke?branch=master
+    :alt: See coverage stats on Coveralls
 
 =======
 nbsmoke

tox.ini

@@ -1,12 +1,24 @@
 [tox]
 envlist = py27,py36,py37,py38,pypy,flake8,dev
 
-# TODO: shouldn't be 'all' in every case.
+# TODO: shouldn't be extras=all in every case.
 
 [testenv]
 commands = py.test -v {posargs:tests}
 extras = all
 
+[testenv:coverage]
+setdevelop = True
+passenv = TRAVIS TRAVIS_*
+# don't use pytest-cov as nbsmoke is itself a pytest plugin, so things
+# get confusing (e.g. nbsmoke gets imported before coverage collection
+# starts, so all the module-level code is missed)
+commands = coverage run --source=nbsmoke -m pytest
+           coverage report -m
+           coveralls
+extras = all
+deps = coveralls
+
 [testenv:flake8]
 skip_install = true
 deps = flake8