Use pathname to discover full path for relative path includes

This changes allows us to use relative paths in include files in the config, for example:
```
:include:
    - "../sec_groups.yaml"
    - "../common-mgmt.yaml"
    - "instances.yaml"
```

Results in output during run:
```
$ bundle exec build-cloud -c environment/euwest1-uat.yaml --create --find-type security_groups
I, [2014-10-09T07:55:18.991012 #26158]  INFO -- : Including YAML file /srv/data/repos/customer/repository/build-cloud/sec_groups.yaml
I, [2014-10-09T07:55:18.991908 #26158]  INFO -- : Including YAML file /srv/data/repos/customer/repository/build-cloud/common-app.yaml
I, [2014-10-09T07:55:18.992752 #26158]  INFO -- : Including YAML file /srv/data/repos/customer/repository/build-cloud/environment/instances.yaml
```

Which allows us to resolve paths that may be referenced with `.` or `..`
Without this change, any path referenced with `..` or `.` is not resolved correctly, and the config file referenced is ignored