Add description of variables in default template

List of keys available in nginx server configuration has been moved from
'defaults/main.yml' file directly into 'default.conf.j2' template, each
parameter has its own description.
Merge pull request #8 from drybjed/move-defaults

Add description of variables in default template
Remove readme text from meta file, convert synopsis to Markdown
Replace reStructuredText README with Markdown README