replace deprecated opt parse by argparse

cf. PEP 389
add positional argument "inputfile" to argparse
convert input file to output file only if latter is outdated
avoid list comprehension shadowing previous p definition
comment unused vars detected by pyflakes
add command line option --force

to convert a dot file even when the output tex file exists and is newer.
This serves for enforcing changes of dot2texi command options.
Remove mutable default argument + minor PEP8 work
Remove unused start_fig and end_fig methods
Coordinates are floats in general - but you do know that.

It really is an annoying bug :-) It stopped my entire borders of some elements (subgraphs or - I haven't checked yet for that being corrected by the change - nodes) not to be drawn.
Merge pull request #31 from VABAV/patch-1

Convert parsed polygon, polyline and B-spline coordinates to float.

Close issue #31
Convert parsed ellipse coordinates and dimensions to float instead of int
Update change log
Add test for issue #28
Relax regexp for finding include files to allow comments

Close issue #28
Add test for issue #23
Add tests for head and tail label with the pstricks format
Add tests for head and tail labels in the duplicate mode
Add head and tail label support in duplicate mode and in the pstricks output format
Update change log
Remove unneeded default False values

dict.get(key) returns None as default, which is a falsy value