merge in several improvements to the plugin

Hello, 

Thanks for the original redmine_wiki_sql plugin. Found it useful, and added several needed improvements
- add security features so it's safe to use in production: (1) allow configuring DB separately for wiki_sql (via the admin gui) so you can create a less-privileged DB user, instead of reusing the rails connection (2)
- fix the column ordering/naming bug
- change the syntax, which improves quite a bit: (1) no more having to escape parentheses (2) use as much whitespace as you need to make SQL query pretty (3) select a specific db per query to save time writing table names
- automatically format returned text in textilized format (useful when listing stuff like issues, etc)

With that said I also added two flaws (1) now it can only really work with MySQL (2) if an authorized SQL user foolishly writes a query that returns the very same text of the query, you can end up with an infinite query and crash redmine

At least for my uses, adding the security alone made it worth it (since I'm only using it with MySQL) and really would not have been able to use it at all otherwise. But YMMV and some users (especially if not using MySQL!) might not find the other improvements worth the downside

Thanks again for the original plugin and open sourcing it!