refactored _util.make_fields() to field_type() and code cleanup
refactored ArrayType to use field_type()
code cleanup
refactored FixedWidthWriter
refactored tests in test_writer.py
_Reader.filter() accepts multiple arguments
tweaked test/test_reader.py
_Writer.filter() accepts multiple arguments
added variable-length array tests
variable-length fields are initialized with a width of None
refactored _TabularWriter hierarchy
replaced _util.make_field with Field class
added ArrayType.width attribute
_TabularWriter dynamically updates array field positions and widths
code clean-up and additions for tutorial.md
v0.1.3