Support the Graphviz size attribute

[kjellmf]

Dot2tex currently does not do anything with the size attribute. This is how
Graphviz interprets size:

Maximum width and height of drawing, in inches. If defined and the drawing
is too large, the drawing is uniformly scaled down so that it fits within
the given size.

If size ends in an exclamation point (!), then it is taken to be the
desired size. In this case, if both dimensions of the drawing are less than
size, the drawing is scaled up uniformly until at least one dimension
equals its dimension in size.

A similar behavior could be implemented by scaling the resulting
tikzpicture or pstrics environment.

Original GC issue: https://code.google.com/p/dot2tex/issues/detail?id=7