JIRARenderer: Fix empty lines between block elements and more

[pbodnar]

Like #97, this mainly removes empty lines within lists for their proper Jira rendering.

It also fixes other JIRARenderer problems, so that the renderer is basically usable for real-world scenarios. In short, the main fixes (see individual commits for more details):

• Properly separate paragraphs and others by an empty line.
• Line as well as block quotes are generated.
• Don't HTML-escape all the text. (or please explain why to do it)
• The intended outputs are covered by unit tests which can be easily extended by following a simple naming convention.

Note that I'm rather a Python newbie, so don't hesitate to correct me to possibly better follow the Python-style coding.

[coveralls]

Coverage increased (+1.7%) to 95.007% when pulling 0daa6058db816a1c7a6391f03185f764210f5003 on pbodnar:fix-jira-renderer into 2cfe7446b975685f98837f9e40aaabcc0e270a79 on miyuchina:master.