Major improvements to DIMACS, parsing

Created a new package pyeda.parsing that contains two modules, lex.py
and token.py that should be common between modules that implement
parsers.

The "lex" module (lex.py) contains the RegexLexer class, which
implements a somewhat lex-like tokenizer based on regular expressions.
It was at least partially inspired by the design of the RegexLexer class
in pygments.

Used the new common code to implement both DIMACS CNF and SAT parsers.

Unfortunately, I had to ignore the pyeda.parsing package for PyLint,
because it blows up with some odd AttributeError.

Fix issue #27 (DIMACS too complicated)