Make Responses context managers.
Document Responses as context managers.

Closes #24.
Test resizing Encoder header table.
Let TypeErrors bubble up.

We control the inputs to this class well enough that it should be very
difficult to hit this error handler: even if we do, we don't want to let
this pass without comment.
Define new HPACKEncodingError exception.
Test removing headers partially in the table fails.
Test removing totally absent value fails.
Document window managers.
Test header table resizing in Decoder.
Add chunked reads to DummyStream.
Test reading small chunks from response.
Test buffered reads on responses.
Clearer assertion.
Test Response.getheader().
Explicitly show that fileno isn't implemented.
Covering the TLS file is hard and unnecessary.

These branches are difficult to test and prove nothing.
Test clean shutdown.
Define the ConnectionError.
Test abnormal shutdown.
Test receiving unexpected frames.