include: update uv_backend_fd() documentation

uv_run_once() is no more, replace with uv_run(UV_RUN_NOWAIT).

Fixes #759.
unix: include uv.h in src/version.c

Include uv.h so the compiler sees the right visibility attribute for
uv_version() and uv_version_string().

GYP builds compile with -fvisibility=hidden. Before this commit, the
symbols were not visible in libuv.so.

Fixes joyent/node#5213.
Merge remote-tracking branch 'origin/v0.10'