Remove simplejson fallbacks in tests: we don't support Python < 2.6
Update the tests readme to be accurate.
Google Code Issue 215: Properly detect seekable streams

Removes the hack that tests for sys.stdin to determine if the stream
is seekable (it has tell() and seek() but it is not seekable) by
actually calling tell() and seek().

This also removes the wrapping in HTMLUnicodeInputStream, where we
do not need the ability to seek.