Simplify encoder

Remove use_decimal, namedtuple_as_object, tuple_as_array and item_sort_key from PBJSONEncoder constructor
Remove use_decimal, namedtuple_as_object, tuple_as_array, item_sort_key, ensure_ascii, allow_nan, cls, indent, encoding, ignore_nan and bigint_as_string from dump and dumps
Implement pure python decimal encoding for float and decimal
Allow sort_keys to be either True or a callable
Add tuple check to is_namedtuple in speedups
Handle objects with a length better in speedups
Simplify nan and inf detection