Fixed bugs in parsing machinery

1) Bug fix: Added  import to 'grammar_utils class'
2) Updated referenced and methods in class 'run-syntax-tests'. Tested
with all test-cases (unresolved bugs have been reported in the google
group).
3) Bug fix: method '_store_stmt' of 'grammar' class modified to support
function definitions that have no arguments but a return type (e.g.,
function[Y]=valid_51).
Added testing framework for converter class

1) Added a converter-tests-cases folder under converter with matlab
examples containing ODE models with different kinetics (more will be
added later).
2) Also added a run-converter-tests.py class that emulates the testing
framework in place for the parser.