Get config parameter AllCops/Excludes from highest config file in path.

Look upwards towards root for a .rubocop.yml and take Excludes value
from highest file. This solves the problem of .rubocop.yml files in
directories that were supposed to be excluded shadowing the file that
excludes them.

Also fix inherited relative Excludes paths so they are relative to the
.rubocop.yml file that inherits them.
Update CHANGELOG and README.
Merge pull request #295 from jonas054/fix_exclude_problem

Get config parameter AllCops/Excludes from highest config file in path.