Ignore any unused entry that starts with '_'.

This makes it possible to follow PEP8 which recommends '__',
and to name unused entries, e.g.:

    foo, _bar, _zed = func()
'unused' prefix to suppress warnings of unused variables

In addition to '_', also ignore any unused variables whose name start
with 'unused', as recommended by http://google-styleguide.googlecode.com/svn/trunk/pyguide.html?showone=Lint#Lint
Merge pull request #295 from andreasvc/master

Ignore any unused entry that starts with '_'.