Handle an escaped newline at the end of a physical line

This fixes https://github.com/jcrocholl/pep8/issues/319. I've added some tests, checked that 'make test' passes, and checked that pep8.py still validates itself.

I had to correct one testcase in W19.py - a tab-indented physical line ended in a backslash, so didn't previously undergo physical line checking and didn't raise W191.