Add mode for displaying step numbers inside tooltip section

I thought it might be useful to add another mode for step numbering, which allows displaying the number inside the tooltip section, as follows:

![image](https://f.cloud.github.com/assets/639935/758632/07297f0e-e744-11e2-81d1-aac2a859a1c5.png)

This has three advantages (that I could think of):
* Step numbers are still visible in case of elements that are at the top of the page.
* Users can see how many steps they have left
* The visualization is more subtle - it doesn't take much attention but still very useful.

There are three possible modes that can be set via the 'showStepNumbers' option: 'none', 'inset' and 'outset'. 'none' will disable numbering, 'outset' is the default mode (as it was before) and 'inset' is the new mode (as can been seen in the image above). 

My changes are completely backwards compatible: true is translated into 'outset'; false into 'none'.