Fix sass#775: Introduce SASS_BINARY_SITE environment variable

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

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

If SASS_BINARY_SITE is set to

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

then

 http://myhost:8080/local/node-sass-bin/v3.0.0-beta.5/freebsd-x64-14_binding.node

should point to the FreeBSD 64 bit binary for node 0.12.0

The URL can be also specified as the --sass-binary-site
commandline option or in the package.json:

 "nodeSassConfig": {
   "binarySite": <url>
 }

While here, fix operator precedence for --sass-binary-url.

lib/extensions.js

@@ -50,7 +50,7 @@ function getRuntimeInfo() {
 /**
  * Get binary name.
  * If environment variable SASS_BINARY_NAME or
- * process aurgument --binary-name is provide,
+ * process argument --binary-name is provided,
  * return it as is, otherwise make default binary
  * name: {platform}-{arch}-{v8 version}.node
  *
@@ -81,16 +81,43 @@ function getBinaryName() {
  * is set, return that path. Otherwise make
  * path using current release version and
  * binary name.
+ * 
+ * Optionally, the default URL can be replaced by
+ * setting the environment variable SASS_BINARY_SITE
+ * (or commandline option --sass-binary-site)
+ * which should point to the mirror of the repository
+ * laid out as follows:
+ *
+ * SASS_BINARY_SITE/
+ * 
+ *  v3.0.0-beta.4
+ *  v3.0.0-beta.4/freebsd-x64-14_binding.node
+ *  v3.0.0-beta.5
+ *  v3.0.0-beta.5/freebsd-ia32-11_binding.node
+ *  v3.0.0-beta.5/freebsd-x64-42_binding.node
+ *  v3.0.0-beta.5/freebsd-x64-14_binding.node
+ *  v3.0.0-beta.5/freebsd-x64-11_binding.node
+ *  v3.0.0-beta.5/freebsd-ia32-43_binding.node
+ *  v3.0.0-beta.5/freebsd-ia32-42_binding.node
+ *  v3.0.0-beta.5/freebsd-ia32-14_binding.node
+ *  v3.0.0-beta.5/freebsd-x64-43_binding.node
+ *
+ * ... etc. for all supported versions and platforms
  *
  * @api private
  */
 
 function getBinaryUrl() {
   return flags['--sass-binary-url'] ||
-         package.nodeSassConfig ? package.nodeSassConfig.binaryUrl : null ||
+         (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('');
+         [
+           (
+             flags['--sass-binary-site'] ||
+             (package.nodeSassConfig ? package.nodeSassConfig.binarySite : null) ||
+             process.env.SASS_BINARY_SITE || 'https://github.com/sass/node-sass/releases/download'
+           ), 'v' + package.version, sass.binaryName
+         ].join('/');
 }
 
 /**