Addresses #697.

Fixed lint error which was causing tests to fail.

CLI: Adds support for multi-file compilation.

Code: Skips path.resolve when outFile is not set

Code: Don't throw from interim callback state

Lets the error propagate to libsass.

This goes against the concept of fast aborting,
but this way downstream user will not get a blunt
exception, but receive a well-formatted JSON
error.

Note: this exception from importer callback's
post-process handler cannot be captured by try-
catch.

The edit in binding.cpp is also required, because
those conditions will never get satisfied anyway.
Removed due to aforementioned reasons.

Importer: Throws error on invalid return type

Issue URL: #670.
PR URL: #833.

API: Provides support for array of importers

* In order to skip the importer, user must
  return `sass.types.NULL` (or the shorter
  alias `sass.NULL`).
* See the added test on usage.
* Backward compatible: part of me want still
  wants to make it non-backward compatible
  because:
  * We can: in major version v3.0.
  * It will keep the API clean.
  * It will keep the docs clear: type of
    `options.importers` is array of functions.
* Updates docs.

Issue URL: #821.
PR URL: #832.

* Also updates dependencies versions.

ad br so readme cli options differentiate

Build: Adding libsass.gyp in a separate dir

* Turned out it isn't a bug, it's a feature!
  * npm/npm#7780

* Because we tend to fix them, not ignore.

* Adds corresponding tests.

Issue URL: #651.
PR URL: #817.

CLI: Adds support for Custom Functions

* Adds related tests and fixtures.

Issue URL: #784.
PR URL: #815.

Feature: Provides sourceMapRoot option

Issue URL: #776.
PR URL: #813.

Code: Improves binary test

* Removes slash from URL.

Code: Reverted #763

Minor cleanup; removing redundant headers
from `.cpp` files.

Removes upload.js, since it should live in
node-sass-binaries repo.

Issue URL: #802.

Code: Mics. formattings

Feature: Configurable linefeed and indent