Switch JavaScript engine from js2py to dukpy

[carsonyl]

This changes PyPAC to use dukpy instead of js2py for its JavaScript engine. This simplifies PyPAC's API and should eliminate memory usage concerns. A few months ago, dukpy started distributing wheels for Windows in addition to Mac and Linux, which is great timing.

• Removed recursion_limit argument on pypac.PACSession and pypac.parser.PACFile. Usage now results in a deprecation warning. This behaviour is not configurable in dukpy, and should not be a problem in practice.
• Deprecated pypac,parser.PyimportError and pypac,parser.PacComplexityError. These no longer get raised. Usage now results in a deprecation warning.
• pypac.parser.MalformedPacError conveys the dukpy exception that it masked.

Expected to fix #20 and #22.

[coveralls]

Coverage decreased (-2.8%) to 88.936% when pulling 0dac867f57efb327085d592c0bca990207e2caa3 on dukpy into 8e5091fbb5bf53bebc8a28688d060de46063f5a7 on master.