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

Turns our handling proxies is complicated. The sanest thing to do
is try matching [npm's behaviour][1], and liable [to change][1] to
our benefit. The TLDR; of which is to let `request` do whatever
it wants unless npm has been explicitly configured otherwise.

Fixes #1724

[1]: https://github.com/npm/npm/commit/40afd6aaf34
[2]: https://github.com/npm/npm/issues/7168

Fix process argument binary configuration example

Should be --sass-binary-site instead of --SASS_BINARY_SITE

This was removed #1733 but was depended on by #1714.

User configurable cache folder for binary download

Adding unit test coverage to install

The installer is getting more complicated. The core of it is a http
request which makes testing it difficult/messy. Pulling out core
logic into small util functions that we can unit test would give me
much more confidence moving forward.

I would prefer we don't export functions from scripts so people don't
start relying on undocumented APIs. This is why I opted for helper
utils but I'm not committed to them.

New order of operations
1. Look for existing binary in vendor folder
2. Create target vendor folder
3. Look to see if we’ve cached a copy in the configured or NPM cache
4. Download to current version’s /CACHE/node-sass/version/binding_file
5. Copy the cached download to the regular vendor directory
Closes #1566

Node 5 is neither latest or LTS. This is not removing support for
Node 5, it's simpky removing it from day-to-day CI for faster builds.