Make all tutorial snippets pass

[brson]

These run as part of the test suite now but there are failures, primarily for three reasons: #1501, some task deadlocks, a dependency on libcrypto.

[marijnh]

I think the ffi example requiring libcrypto is okay. The float::const::pi failures should be fixed now (lht pushed a fix today, I think). I've never seen the deadlock, though.

[brson]

I unxfailed the pi examples and modified the comm examples so they don't wait forever. The only thing left is the crypto examples. Last time I turned them on they failed on the linux bots.

[thestinger]

Closing in favour of #4509. libsnappy.so would need to be available on the buildbots, but only some of the samples are standalone/testable.