Fix CLI usage

Updated CLI usage as per #669

Replace "request" by "got"

* Less dependency
* Use only stream

Release 3.5.0-beta.1

No need to wrap the section in a conditional

Use /RELEASE option of LINK.EXE

To ensure proper debugging of problems of Windows,
publish PDB debug file along with the binary.

In order to keep paths to the source files clean apply
the technique described by John Robbins in
"PDB Files: what every developer must know"
http://www.wintellect.com/devcenter/jrobbins/pdb-files-what-every-developer-must-know
and build node-sass in the "S:\node_modules\node-sass"

During debugging, source files can be referenced
via the virtual "S:" drive.

This allows users and developers to assign S: drive to the project
root ("node_modules") and access source files without having to redefine
source paths.

Artifact names are generated in a nice, Lisp-styled functional PowerShell
hidden in a YAML folded scalar!

Add node 5 to the PR testing build.

Revert "Bump AppVeyor to use Visual Studio 2015"

Disables Thread-Safe for local static variables

Fixes #1283.

This reverts commit 901f4114.

Bump to latest libsass

Normalize the way of writing 'LibSass' to the one used by the project

Accept process.execPath as-is

Sourcemaps got an overhaul in LibSass 3.3.3. Given the current
state of sourcemaps let's assume the new mappings are an
improvement.

This is just to see what CI thinks

Fix detection of error specs

Fix false positive when spec has a subfolder called error

Update license year range to 2016

Remove red tape surrounding process.execPath
and use it as-is. In particular, do not try
to canonicalize the path returned by
process.execPath as this might be relative
in the restricted Linux environments
(for example using overlayfs).

We should be prepared to accept relative
or otherwise broken process.execPath and
have a faith it works (otherwise our
build scripts wouldn't be invoked
in the first place).

https://github.com/sass/node-sass/issues/1323

Allow symlinked output directories

Bump npm deps

get-stdin wasn't bumped to ^5 because it introduced a breaking
change which would require use to drop support for node 0.10 or
add a promise library just for this module.

Update contributing docs.

Handle sass.NULL returned from the importer via done()

Whenever done() is called we should check for the sass.NULL
value as well.

While here, allow calling done(null) to anticipate
removal of the sass.NULL one day.

Fixes https://github.com/sass/node-sass/issues/1296

Add test coverage for existing custom importer return semantics

Custom importers are expected to return `sass.NULL` to noop.
However it appears we currently also allow `null`, `false`, and
`undefined`.

This patch adds test coverage over these caveats in case we
unintentionally break BC.

Less V8 bureaucracy for Nan::ThrowTypeError

Use assert.strictEqual in tests for better error output

Use https:// to check out libsass code

When the test fails, we are getting actual
values in the error message instead of

+ true
= false