Update node-webkit to 0.10.0

`link` stanza has been removed as it does not serve normal usage of
node-webkit.

node-webkit is a runtime targeted by application developers.
Usage of node-webkit usually takes one of two forms:
- launching the binary with development code as argument, either by
  CLI or drag-and-drop.
- bundling a copy of the binary with release code, for packaging and
  distribution.

Neither usage is aided by a symlink; a known binary location, which
Cask provides, is sufficient.

Addendum: `version` value is reused in `url` stanza to facilitate
future updates.

Casks/node-webkit.rb

@@ -1,9 +1,9 @@
 class NodeWebkit < Cask
-  version '0.9.2'
-  sha256 '2133f498d2607e02a1af1b1988fb191ed0467a268afffabbeb3b99ee62384407'
+  version '0.10.0'
+  sha256 '37e2000b5bbc9a96c55ab15c86b21aeb38de2aa7d373c04c5f820d5bae07dc54'
 
-  url 'http://dl.node-webkit.org/v0.9.2/node-webkit-v0.9.2-osx-ia32.zip'
+  url "http://dl.node-webkit.org/v#{version}/node-webkit-v#{version}-osx-ia32.zip"
   homepage 'https://github.com/rogerwang/node-webkit'
 
-  link 'node-webkit.app'
+  caskroom_only true
 end