Fix bug in big_endian property of WKBWriter (issue #174)

See https://github.com/Toblerity/Shapely/issues/174

WKBWriter did not correctly interpret its `big_endian` property. It
wrote WKB in big-endian byte order when big_endian=False, and in
little-endian byte order when big_endian=True.

This commit fixes this bug and adds basic test coverage in a new
test_wkb.py module.