Fix bugs in calls to add_offence.

There are a number of places where the location argument to add_offence
is still the line number. It's supposed to be a location object that
responds to line and column. The way to catch these errors in our unit
tests is to access line and column in the Offence constructor.
Print relative paths if the paths are under the working directory

This affects to the simple/progress formatters.
Merge pull request #262 from yujinakayama/print-relative-path

Print relative paths with the simple/progress formatter
Merge pull request #261 from jonas054/fix_location_bug

Fix bugs in calls to add_offence.