Allow setting one of the limits of scale

Implements setting of only one limit of a continuous scale by specifying
NA for the limit which you don't want to set.
Style fixes suggested by winston
Update news and update documentation
Reformat news
Fix documentation for ylim

The range of mtcars wt data is from 1.513 to 5.424, so the previous example was
causing an error because there were no points to obtain a range from.
Merge pull request #908 from jimhester/one_limit

Allow NA in limits