Use \setkeys{Gin}{} to set appropriate defaults for \includegraphics, instead of entirely redefining this command.

Note we also consider the height of images; if an image is too high, 0.8\textheight will be used for beamer slides, and \textheight is used for articles (see http://tex.stackexchange.com/q/11954/9128).

If the user has explicitly provided the width/height options in \includegraphics[], our defaults will be overwritten, and this approach is better than the check `\@ifnextchar[` after \includegraphics because the latter approach simply gives up everything once it sees [, whereas \setkeys{Gin} can keep the good defaults unless they are explicitly overwritten in [].

closes #30, closes #28, and closes #26
Merge pull request #58 from yihui/master

Use \setkeys{Gin}{} to set appropriate defaults for \includegraphics