Structural parser

Basic parser, using http://www.dabeaz.com/ply/

Seeking a CR regarding python style and idioms, rather than technical review of ATF format parser, so no need for reviewer to read the documentation at 

http://build.oracc.org/doc2/help/editinginatf/primer/index.html
and
http://build.oracc.org/doc2/help/editinginatf/primer/structuretutorial/index.html

unless you want to.