Fix failing specs that break in PathUtil.relative_path

When Include and Exclude configuration parameters were added in
default.yml, some specs could no longer run with the default buffer
name "(string)" or "example.rb". They need an absolute path.

Fixing this revealed some other problems; that app/models must be changed to app/models/*.rb for example. Also, the paths given in Include are
not interpreted as relative to the configuration file. They should be.
Merge pull request #891 from jonas054/fix_include_config

Fix failing specs that break in PathUtil.relative_path