instructions and tests for building and testing node-sqlite3 on node-webkit

There is [node-webkit](https://github.com/rogerwang/node-webkit). It runs Node modules in the context of a WebKit-based (to be more precise, Cromium-based) web browser's window(s), and thus the methods of web development can be used to write cross-platform GUI applications (for Windows, for Mac, for Linux) in JavaScript.

Quite fortunately, node-sqlite3 may run on both engines (Node.js and node-webkit) without any changes in its source code.

However, the ABI of Node.js and node-webkit are different, and thus another tool (nw-gyp instead of node-gyp) should be used to build the node-sqlite3 module for node-webkit. An alternative approach to testing is necessary as well.

I am contributing the instructions and a test wrapper necessary to build and test node-sqlite3 for node-webkit.