jinja: Remove Twig compatibility, add timer, improve code. - Add a timer for keeping track of extensions' performance. - Use the `select_template` function from Jinja to cut down on a few lines of code.

cov-core==1.15.0
coverage==4.0.3
invoke==0.12.2
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1