Adds a setting to cache textile rendering (off by default).

Adds a setting to cache textile rendering (off by default).
* it uses ActionController cache store which is MemoryStore by default and can be configured with config.action_controller.cache_store
* macro processing was moved out of textile rendering so that it doesn't get cached
* no noticeable improvement is expected for small portions of text, so only texts larger than 2KB are cached

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3372 e93f8b46-1217-0410-a6f0-8f06a7374b81