refactored StringField tests
refactored field.py module
fix for ConstField with default formats
improved RecordField tests
renamed test scripts
renamed test/ directory to tests/
modified test execution
moved requirements-py26.txt to tests/
fixed Travis CI file
code clean-up in setup.py
fixed sample code in tutorial.md
restructured test package
test code clean-up
added material to README.md
fixed .travis.yml
setup.py fixes and refactoring
added optional grouping to SortReader/Writer
tweaked _Sort class
SortReader/Writer fix for multi-key sorts
improved tests