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

* `indentType` can be space or tab.
  * default: space.
* `indentWidth` is a positive integer.
  * default: 2.
  * maximum: 10
* `linefeed` could be:
  * cr
  * crlf
  * lf
  * lfcr

Issue URL: #682.
PR URL: #792.

malloc() needs <stdlib.h>

Custom functions

Submodule: Updates LibSass to v3.2.0-beta.1

* Updates sass-spec to corresponding commit.
* Prevents source_string from freeing in
  data context case in binding code.
  * Corresponds to sass/libsass#925.

Code: Parses arguments manually

Build with the unbundled libsass

Use environment variables to control
how libsass is linked:

LIBSASS_EXT variable empty or unset

Build a bundled libsass source
(from src/libsass).
This is the default.

LIBSASS_EXT=auto

libsass is located using pkg-config
and linked dynamically

LIBSASS_EXT=yes
LIBSASS_CFLAGS=    compiler flags
LIBSASS_LDFLAGS=   linker flags
LIBSASS_LIBRARY=   library to link

Link libsass manually, by adding
compiler and linker flags.

For example this way libsass can
be linked in statically (on Unix):

LIBSASS_EXT=yes
LIBSASS_CFLAGS=-I/usr/local/include
LIBSASS_LIBRARY=/usr/local/lib/libsass.a

When using LIBSASS_EXT there is no need
to checkout the libsass source into src/libsass submodule
and the library from the package management
system can be used.

PR: #139
PR: #389
PR: #744

Issue URL: #769.
PR URL: #770.

CLI: Do not require input file on watch

* Removes input file parameter from watch test.
* Uses `once` instead of `on` in CLI tests.
  * Removes `bin.kill` in favour of `once`.

Issue URL: #755.
PR URL: #761.

Repo: Adds .gitattributes