Centralised and improved rule texts. Paired by Geoff and Michael.

The rule texts have been placed in a central mako file where they can be
drawn from by all files. In this way, we must only change one thing to edit
the text instead of multiple files. Also, the language used in the text was
improved. (Feature #133)
Merge pull request #44 from CO600GOL/feature/#133_rule_texts

Centralised and improved rule texts. Paired by Geoff and Michael.