Issue #166 yaml removing underscores

This fixes #166 (closed) and adds tests

buildstream/_project.py: Changed max-jobs to initialise as a string

buildstream/_variables.py: Changed max-jobs to initialise as a string

buildstream/_yaml.py: Overrode the ruamel constructor to treat everything as a string

tests/context/context.py: Removed old, outdated test and added new related test

tests/context/data/convert_value_to_str.yaml: Created this testing file

tests/context/data/invalidtype.yaml: Removed this outdated testing file