Fix sass#775: Introduce SASS_DIST_SITE environment variable

Provide ability to locally mirror node-sass
binaries for various versions and platforms.

SASS_DIST_SITE needs to be an URL pointing to
a collection of files organized like the Github
repository.

If SASS_DIST_SITE is set to

 http://myhost:8080/local/node-sass-bin

then

 http://myhost:8080/local/node-sass-bin/v3.0.0-beta.4/freebsd-x64-14_binding.node

should point to the FreeBSD 64 bit binary for node 0.12.0

lib/extensions.js

@@ -89,8 +89,9 @@ function getBinaryUrl() {
   return flags['--sass-binary-url'] ||
          package.nodeSassConfig ? package.nodeSassConfig.binaryUrl : null ||
          process.env.SASS_BINARY_URL ||
-         ['https://github.com/sass/node-sass/releases/download/v',
-          package.version, '/', sass.binaryName].join('');
+         [
+          (process.env.SASS_DIST_SITE || 'https://github.com/sass/node-sass/releases/download/'),
+          'v', package.version, '/', sass.binaryName].join('');
 }
 
 /**