Png depiction changes

Hi,

I made a bunch of changes to the png depiction to fit my needs and I thought I would share them... I'm not very experienced with pull requests, so maybe I haven't done everything optimally, for example if you would like me to split the commits into different requests, please let me know.
The -xm option added in a509587 was difficult to properly explain in a few words but basically now you always get a png with the set height and widths, regardless of the compound. With this change, single molecule depictions can be made dependent on the shape of the compound, so that there won't be a lot of empty space around. I'll try to attach two example images to make it easier to understand... (The margins aren't that easy to see, but if you download the images it's more apparent).
![without_xm](https://f.cloud.github.com/assets/86855/1001311/1b1a82a6-0a67-11e3-8fa2-18b76fc79cb5.png)
Created with `obabel -ismi -:"[O-][C@@]([H])(C)COc1ccccc1[NH3+].[Cl-] test" -xd -O ~/Desktop/without_xm.png`
![with_xm](https://f.cloud.github.com/assets/86855/1001312/2146d9fe-0a67-11e3-82f1-96998a5097ce.png)
Created with `obabel -ismi -:"[O-][C@@]([H])(C)COc1ccccc1[NH3+].[Cl-] test" -xm -O ~/Desktop/with_xm.png`