Auto-PING idle websocket connections

<a href='#crh-start'></a><a href='#crh-data-%7B%22processed%22%3A%20%5B%22https%3A//github.com/cesanta/net_skeleton/pull/46%23issuecomment-61404629%22%2C%20%22https%3A//github.com/cesanta/net_skeleton/pull/46%23issuecomment-61404951%22%2C%20%22https%3A//github.com/cesanta/net_skeleton/pull/46%23issuecomment-61405019%22%5D%2C%20%22comments%22%3A%20%7B%22General%20Comment%22%3A%20%7B%22html_url%22%3A%20%22https%3A//github.com/cesanta/net_skeleton/pull/46%23issuecomment-61404629%22%2C%20%22comments%22%3A%20%5B%7B%22body%22%3A%20%22Can%20you%20please%20rebase%20this%20branch%20against%20master%20and%20force%20push%20so%20that%20we%20don%27t%20have%20too%20many%20spurious%20merge%20points%20in%20our%20history%3F%20%28it%20makes%20git%20log%20--graph%20cleaner%29%22%2C%20%22created_at%22%3A%20%222014-11-02T12%3A22%3A02Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/52673%3Fv%3D2%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/mmikulicic%22%7D%7D%2C%20%7B%22body%22%3A%20%22Done%22%2C%20%22created_at%22%3A%20%222014-11-02T12%3A34%3A34Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/601816%3Fv%3D2%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/cpq%22%7D%7D%2C%20%7B%22body%22%3A%20%22%3Ashipit%3A%22%2C%20%22created_at%22%3A%20%222014-11-02T12%3A36%3A45Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/52673%3Fv%3D2%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/mmikulicic%22%7D%7D%5D%2C%20%22title%22%3A%20%22General%20Comment%22%7D%7D%2C%20%22approved%22%3A%20%7B%22https%3A//github.com/mmikulicic%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/52673%3Fv%3D2%22%7D%7D%7D'></a>
<a href='https://www.codereviewhub.com/'><img src='http://www.codereviewhub.com/site/github-bar.png' height=40></a>

<img src='http://www.codereviewhub.com/site/github-approved-avatar.png'><a href='https://github.com/mmikulicic'><img src='https://avatars.githubusercontent.com/u/52673?v=2' width=34 height=34></a>

- [x] <a href='#crh-comment-General Comment'></a> <img src='http://www.codereviewhub.com/site/github-completed.png' height=16 width=60>&nbsp;<b><a href='https://github.com/cesanta/net_skeleton/pull/46#issuecomment-61404629'>General Comment</a></b>
- <a href='https://github.com/mmikulicic'><img border=0 src='https://avatars.githubusercontent.com/u/52673?v=2' height=16 width=16'></a> Can you please rebase this branch against master and force push so that we don't have too many spurious merge points in our history? (it makes git log --graph cleaner)
- <a href='https://github.com/cpq'><img border=0 src='https://avatars.githubusercontent.com/u/601816?v=2' height=16 width=16'></a> Done


<a href='https://www.codereviewhub.com/cesanta/net_skeleton/pull/46?mark_as_completed=1'><img src='http://www.codereviewhub.com/site/github-mark-as-completed.png' height=26></a>&nbsp;<a href='https://www.codereviewhub.com/cesanta/net_skeleton/pull/46?approve=1'><img src='http://www.codereviewhub.com/site/github-approve.png' height=26></a>&nbsp;<a href='https://www.codereviewhub.com/cesanta/net_skeleton/pull/46?approve=0'><img src='http://www.codereviewhub.com/site/github-undo-approve.png' height=26></a>&nbsp;<a href='https://github.com/cesanta/net_skeleton/pull/46'><img src='http://www.codereviewhub.com/site/github-refresh.png' height=26></a>
<a href='#crh-end'></a>