Fixes Issue #55

continuation lines are now calculated from the logical indent level
(1 tab = 8 spaces, as calculated by `expand_tab`)
not the physical number of whitespace characters
(which will only be the same if indents are only defined with spaces and not tabs)

tests pass on python 2.7 at least, it's only a one-line change, but it's dependent on behavior of the `tokenize` module