Looks the default of stderr is causing issue for some CI servers.

Fixes #1801

I'm not sure why the throw isn't being caught. Probably some async
thing. Don't have time to look into it atm.

Handle permission errors when attempting to cache binaries

This uses a candidate model for determining an suitable cache
directory. This is different from the current approach with eagerly
chooses a cache directory. If that directory isn't accessible for
some reason (i.e. permissions) we have to bail out. With this
approach we can try a prioritised list of candidate directories.

Additionally this address the case where npm cache config is
blank.

Fixes #1781

This reverts commit 5759276e.

Use npmlog for all install script output

In for a penny in for a pound. I also  noticed that #1786 resulted
in some funky looking console output. I've taken this opportunity to
tidy up our install script output whilst maintaining the information
useful for debugging install issues.

```
> node-sass@3.11.3 install /Users/xzyfer/Projects/Sass/upstream/node-sass
> node scripts/install.js

http node-sass install Downloading binary from https://github.com/sass/node-sass/releases/download/v3.11.3/darwin-x64-47_binding.node
http node-sass install Download complete
info node-sass install Binary saved at /Users/xzyfer/Projects/Sass/upstream/node-sass/vendor/darwin-x64-47/binding.node

> node-sass@3.11.3 postinstall /Users/xzyfer/Projects/Sass/upstream/node-sass
> node scripts/build.js

info node-sass build Binary found at /Users/xzyfer/Projects/Sass/upstream/node-sass/vendor/darwin-x64-47/binding.node
info node-sass build Testing binary
info node-sass build Binary is fine

> node-sass@3.11.3 prepublish /Users/xzyfer/Projects/Sass/upstream/node-sass
> not-in-install && node scripts/prepublish.js || in-install
```

Fix binding tests leaking process values

npm config values are strings

Add descendent files to watched files

The state leakage means that running mocha in watch mode causes
random failures.

Closes #1748

Change console.log to npmlog

Fixes #1731

- don't test old Node.JS version on OSX
- don't test iojs, should be covered by 0.12 and 4

Stop messing with require.cache in tests

This removes the need to mess with `require.cache` in tests to test
the unsupported platform error message. I've run into issues a couple
times related to messing with the require cache.

Fix proxy config for binary download

In #1725 we started setting `proxy: ''` to defer proxy resolution
logic to request. However `request` uses `hasOwnProperty` rather
than falsey checks to determine if an option has been set. This
caused `request` to believe were configuring a proxy so it didn't
do it's own proxy resolution.

Fixes #1785
Fixes #1787

Update supported node versions

#1766

This reverts commit b6558523.

Fix links in troubleshooting.md

Respect NO_PROXY when downloading the binary