Emit `close` event from pipeline.

Newer versions of Node.js will send the `exit` event of a spawned
process immediately, even if standard out and standard in have not been
flushed. Rather than try to support older versions of Node.js in a 0.0.1
release of this module, we adopt to Node.js 0.8 behavior and discontinue
testing of Node.js 0.6 at Travis CI.

Closes #24.
Merge branch `close` into `master`.