Fix overlong lines and regularize spacing in geda.py

* Where possible lines over 80 characters where broken (exception:
  the long string templates)
* Spacing of operators was adapted to follow PEP 8
Add silk_circle and silk_arc methods to geda.py

*Please note: I had to guess as to the semantics of the "silk_arc"
 method, it is well possible it will not always produce the desired
 result, regression testing against the kicad generator should be
 done (by someone with access to kicad)