add io.IOBase compatibility to HTTPResponse

Closes #182
add tests for HTTPResponse subclassing io.IOBase
Fix io test for py 3.x compatibility

Also some minor cleanup items
fix response tests for py 2.6
make sure isclosed part of HTTPResponse's closed property gets tested in 3.x
Clean up styling on a comment as suggested by @shazow
adjust HTTPResponse.closed to default to `closed` but properly report coverage in py 3.x
Merge pull request #187 from eteq/httprequest-io-compatibility

HTTPResponse io package compatibility.