fs: add access() and accessSync()

[cjihrig]

fs.exists() and fs.existsSync() do not follow typical node conventions. fs.access() and fs.accessSync() are added as alternatives in this commit.

This is the first part of #103. The second part is actually deprecating exists() and existsSync(). This PR is equivalent to nodejs/node-v0.x-archive#8714

[cjihrig]

For @caineio

1. fs
2. v0.12
3. yes
4. yes

[caineio]

Sorry, but something is not right here:

1. Expected one of: tls, crypto, buffer, http, https, assert, util, streams, smalloc, cluster, child_process, dgram, c++, docs, other, but got: fs
2. validated
3. validated
4. validated

[indutny]

@cjihrig just a moment, I'll add the rest of core modules :)

[caineio]

Hello!

I am pleased to see your valuable contribution to this project. Would you
please mind answering a couple of questions to help me classify this submission
and/or gather required information for the core team members?

Questions:

1. Which part of core do you think it might be related to?
   One of: debugger, http, assert, buffer, child_process, cluster, crypto, dgram, dns, domain, events, fs, http, https, module, net, os, path, querystring, readline, repl, smalloc, stream, timers, tls, url, util, vm, zlib, c++, docs, other (label)
2. Which versions of io.js do you think are affected by this?
   One of: v0.10, v0.12, v1.0.0 (label)
3. PR-only Does make test pass after applying this Pull Request.
   Expected: yes
4. PR-only Is the commit message properly formatted? (See
   CONTRIBUTING.md for more information)
   Expected: yes

Please provide the answers in an ordered list like this:

1. Answer for the first question
2. Answer for the second question
3. ...

Note that I am just a bot with a limited human-reply parsing abilities,
so please be very careful with numbers and don't skip the questions!

In case of success I will say: ...summoning the core team devs!.

In case of validation problem I will say: Sorry, but something is not right here:.

Truly yours,
Caine.

Responsibilities

1. indutny: crypto, tls, https, child_process, c++
2. trevnorris: buffer, http, https, smalloc
3. bnoordhuis: http, cluster, child_process, dgram

[indutny]

@cjihrig should work now ;)

[cjihrig]

For @caineio

1. fs
2. v0.12
3. yes
4. yes

[caineio]

...summoning the core team devs!