remove a questional error handling

as our underlying transport (tcp/ssl) is a stream which doesn't preserve
message boundaries, dropping an unparsable segment likely make the
situation worse.
fix message header parsing

if not enough bytes of data are available, return {error, binary_too_small}
so that the caller can retry when more data come.  it's normal as
our underlying transport (tcp etc) does not preserve message boundaries.
Add unit tests for ofp_parser

We should also test parsing fragments smaller than 8 bytes: see
FlowForwarding/of_protocol#40.
Merge branch 'fix-parser' of github.com:yamt/of_protocol into yamt-fix-parser
Add unit test for parsing 7 byte fragments

This now works!