cut at specific angle feature

Hello,

this pull request should fix #33. I tested it sucessfully with a HDL 32E. We also used this code with an HDL 64E S2. The block-format never changed since 2008 I think. 
A new parameter "cut_angle" is introduced, where one can set the desired "cut angle" in degree in device coordinates.

Standard behaviour is unchanged, so no code should be broken by this fix.

Denis