emscripten / webassembly support

[aoh]

Owl can be built with emscripten, but I/O does not appear to work out of the box.
$ node ol.js -e '(sha256 "trololo")'
Calling stub instead of sigaction()
Calling stub instead of sigaction()
5dd094a5833aba8917bfc7c105eea329143630925f47146bf520c323fbbda1b8
$ node ol.js
Calling stub instead of sigaction()
Calling stub instead of sigaction()
You see a prompt.
[hangs]

[aoh]

Apart from non-blocking operation, the I/O works just fine. This could be fixed as a side product of #42.