Prefer modified date for posts

Use the modified date instead of the original post date if it exists,
but show a flamey hot icon plus the original post date in the mouseover text.
Create variables for date_posted and date_modified

We use these alot, this makes things easier to read.
Replace "Reviewed" with "Updated"

We should be consistent, update actually makes more sense.
Merge pull request #60 from jrha/display_modified_date

Display modified date in lists of posts