Update docs/chapter_0.rst

changed virtualenv --no-site-packages tutorial.todoapp
to ./bin/virtualenv --no-site-packages tutorial.todoapp
(this is to make sure correct virtualenv is used)
changed ./bin/python2.6 bootstrap.py --distribute
to ./bin/python bootstrap.py --distribute
(this is to make it the same command irrespective whether 2.6 / 2.7 is used )
added line to suggest using build-essentials