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.

The basic approach is:
* Count the number of physical lines checked
* Check against the line number to see if all non-blank lines so far have been checked
* Recheck the last line if they haven't

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.