fs: add access() and accessSync()

fs.exists() and fs.existsSync() do not follow the typical
error first callback convention. access() and accessSync()
are added as alternatives in this commit.

Fixes: https://github.com/joyent/node/pull/8714
PR-URL: https://github.com/iojs/io.js/pull/114
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Domenic Denicola <domenic@domenicdenicola.com>
test: split test in parallel/sequential

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/iojs/io.js/pull/172
Fix: iojs/io.js#139
test: run tests in parallel, common improvements

* Allow running tests in mixed parallel/sequential modes
* Add -J flag for running tests on all available CPUs
* Support TEST_THREAD_ID in test/common.js and use it for tmpDir and PORT
* make: use -J flag

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/iojs/io.js/pull/172
Fix: iojs/io.js#139
lib: fix guard expression in timer.unref()

Fixes the following assertion on slow systems, like our ARM buildbot:

    $ out/Debug/node test/simple/test-timers-unref.js
    node: ../src/async-wrap-inl.h:101: v8::Handle<v8::Value>
    node::AsyncWrap::MakeCallback(uint32_t, int,
    v8::Handle<v8::Value>*): Assertion `cb_v->IsFunction()' failed.
    Aborted

The reason it only manifests on slow systems is that the test starts
a 1 ms interval timer, then defers timer.unref.bind({}) to the next
tick.  On fast systems, the test completes in under a millisecond,
before the callback is called.

This commit makes timer.unref() check that the receiver actually has
a timeout callback property.

Fixes #13.