Added a note that python3 is needed.
Added a note that python3 is needed.

When calling mkvirtualenv, the users should choose python3
if not default.
Removed unnecessary notes about unittest2 and python2.7,
python2.6 compatibility.
Removed the note about the requirements_dev.txt completely,
as there is no such file.

glasnt/voc,freakboy3742/voc,cflee/voc,Felix5721/voc,gEt-rIgHt-jR/voc,rubacalypse/voc,rubacalypse/voc,pombredanne/voc,glasnt/voc,gEt-rIgHt-jR/voc,pombredanne/voc,cflee/voc,ASP1234/voc,ASP1234/voc,Felix5721/voc,freakboy3742/voc