Use CSS margin instead of hardcoded offset

[puzrin]

This is follow up for #27 ticket

offset = 10px (by default) can be replaced with margin in CSS. That's seems to be better separation between code & representation.

[stevenbenner]

That is probably the best way to do it. The only part of this task that might be difficult is maintaining backwards compatibility with the way things work now.

I'll have to see if that is possible. If it isn't possible to keep it compatible with existing installs, without hack code, then this is a breaking change and will have to wait for the next major revision.