Add Coveralls coverage testing to CI.

I don't think the number matters that much and I wouldn't want to make
coverage a hard rule for PR's, but I think keeping an eye on the affect
our changes make would help us move in the right direction.

.travis.yml

@@ -18,8 +18,12 @@ before_install:
   - pip install mock
   - pip install unittest2
   - cd $TRAVIS_BUILD_DIR
-install: pip install .[jira,megaplan,activecollab]
-script: nosetests -w tests
+install:
+  - pip install .[jira,megaplan,activecollab]
+  - pip install coveralls
+  - pip install coverage
+script: nosetests -w tests --with-coverage --cover-package=bugwarrior
+after_success: coveralls
 notifications:
     email: true
     irc: