Exit code should be 1 if there is an error in a checks.d or if the flush_count is 0 fix #467
Better info page.

* Display the last 5 logged lines.
* Add a method to the AgentCheck model to create warning messages that will be displayed in the info page.
A useful comment
Use the new warning method for some checks
Remove logged lines in the info page
Remove logged lines from info page
Refactor a bit of stylization code
Merge pull request #469 from DataDog/better_info_page

Better info page