Add if_ready variants of receive_message and poll, when ReadReady trait is implemented for io

[trepidacious]

Fix #39

May help with #36 and #38

[trepidacious]

Just realised there's alaready #35 doing a similar thing - I think that PR will require ReadReady for all connections, for me that led to tests not compiling but I may be missing something. This one adds the new features only if the connection supports ReadReady. On the other hand it might be neater to use the approach from #35 and just let the user access read_ready() to check before doing a receive, rather than building it into a new receive method?

[obabec]

LGTM

[trepidacious]

@obabec Ah great, thanks for reviewing - is there anything else I need to do to allow merging this?