Fix the triangle entity for IE9.

IE9 doesn't understand &blacktriangledown;, so we have to use the
numeric code instead. This at least works cross-platform.

Fixes bug #3017
Fix compatibility with Python 2.5.

The latest RB release broke Python 2.5 compatibility, due to using
@property.setter, which is only in 2.6+. This fixes it to be compatible
with 2.5.

It also sets our markdown dependency to hard-code a working version for
Python 2.5.