use pyyaml for writing metadata instead of ruamel

I've implemented a new approach of writing yaml metadata files.

The basic idea is to transform individual junks of data into YAML and sequentially write them to a file. This way I managed to use pyyaml instead of ruamel.yaml but still enforce the sort order preferred by our metadata maintainers.

I can use this implementation to successfully transform the all txt metadata to yml an back without any significant changes.

This MR changes boolean values. YAML 1.2 (published in 2009) specifies only 2 boolean values: true and false. Older YAML version allowed other more human readable values such as yes which is used in F-Droid metadata up until now.

closes #611