Removed unused HeaderException
Implement SETTINGS frame for draft-13.

- Setting ID is now 16-bit
- Unknown extension settings should be ignored (both on encoding/decoding)
- More tests against invalid input
Implement padding for draft-13

- Remove PAD_HIGH
- Remove padding from CONTINUATION
- Maximum padding length is 256
- Raise ProtocolError on decode error
Ignore unknown/unsupported frame type.
Handle PRIORITY on closed streams
Fix CONTINUATION handling.

On splitting payload for HEADERS, PUSH_PROMISE and CONTINUATION,
frames are not guaranteed to self-contained with regard to HPACK decoding.
One should concatenate payloads prior to decode.
HEADERS should accompary CONITUATION when compressed headers does not fit in one frame.

Connection#encode_headers generates more than one frame when necessary.
More tests on headers splitting.
Implement multiple-valued headers handling.

- On sending, join more than one values for one field name with separators "\0".
- On receiving, do the reverse of the above, except for cookie values are separated with "; ".
- Connection's on(:headers) now receives an Array of key-value array, instead of a Hash.
Update examples
Print ALTSVC when received (but not processed).
Fix ALTSVC test for h2-13
Fix indentation
Fix PONG and its test
Fix connection tests so that actually send HPACK-compressed headers
Priority parameters are sent out on HEADERS and PRIORITY.

Priority-based stream transmission is not implemented yet.
Pass creation options to Connection.new
Merge branch 'stream-h2-13-wip' into draft-13
Merge branch 'hpack-08-wip' into draft-13
Merge branch 'draft-13' into hpack-test-case-08