added SortReader and SortWriter
added RecordField class
changed optional-requirements.txt to requirements-py26.txt [ci skip]
added RangeFilter class
refactored filter.py module
fixed indentation in field.ConstField.__init__() method
added _NumericField base class to field.py
added LongField class
updated Travis-CI configuration
updated README.md [ci skip]
tweaked tutorial.md
v0.3.3