Use git rev-parse to obtain current branch name

Obtaining the current branch name using describe is roundabout and the
tokenization that includes '/' is broken for any branch with '/' in
its name (a common git practice). Using `git rev-parse --abbrev-ref
HEAD` seems to provide the current branch name in an easy to parse
fashion.
Remove unused imports from merl_lib in edown_xmerl
Generated github-style "fenced" code blocks instead of <pre> tags

This reads more like human written markdown, lines up more closely
with the source edoc, and has nicer newline behavior.
Fix "close" of markdown headers, octothorpe counts should match
Adjust newline behavior for generated markdown

Paragraphs begin/end with a single \n. As a result,
fenced code block do not require the extra leading newline and you get
better formatting of ol and ul lists in markdown.