Changed mk_paras to use PARA_RE to split paragraphs from the input text. (PARA_RE honours the triple curly brackets used to denote in-line code).

The previous 'naive' "\n\n" split incorrectly caused a 'break' in the middle of code blocks containing empty lines.
Merge pull request #459 from GazCMarsh/manual-400

Fix for handling of code samples in the manual which contain blank lines.