make runtime renamed to make virtualenv

Default virtualenv directory is ~/.venvs/makeblog for now.
let make test and make coverage use new virtualenv
Move coverage settings from Makefile to .coveragerc
Some more tests
Wipe some more directories on make clean
Workaround for Python < 3.4 in test_templating.

This workaround avoids some race condition. Python < 3.4 has unsorted
test.
Missing imports and test objects.
Some tests for Blog()
Merge remote-tracking branch 'upstream/master'
Added length-option for posts in chronological.html, number of posts rendered can be changed now (via same file