show markdown help on post and comment pages
two small improvements/bugfixes to post form

- fixed bug with "will be posted in /forums" when no forum specified
- changed wording of "what's happening..." when posting in forum
Merge pull request #139 from Skud/markdownhelp

Markdown help, plus tweaks to post form
Merge branch 'dev' of http://github.com/Growstuff/growstuff into dev