detect node-webkit and build with nw-gyp

[maxkorp]

The process here is as such

• Recursively go up the path looking for package.json files.
• When we find the last one, look to see if it has an engines setting, and if it does, look to see if it contains a setting for node-webkit
• If there is a setting, we grab its version
• If we have a node-webkit setting, skip the fetch (we can possibly fix this later?)
• When building, choose between node-gyp and nw-gyp based on having found a node-webkit version
• Set the target for nw-gyp configure to the version found earlier.

[maxkorp]

This fixes #212 (with some caveats) and should resolve other similar issues from coming up. I'm going to make a note in the readme. Going to vote for very tight version support for node-webkit, as that bumps versions (breakingly so) quite often.