doc: fixed isolate declaration and minor cleanups

 * isolate declaration was above `using namespace v8`
 * removed `BUILDING_NODE_EXTENSION` (node-gyp inserts this with a `-D`)
 * minor code cleanups
 * minor reformatting for line length

Is there a good reason we're not using `node::node_isolate` here instead of `Isolate::GetCurrent();`? I'd much prefer to change the recommendation to use it unless there are objections.

I have the examples updated in https://github.com/rvagg/node-addon-examples/tree/v0.11/ (they actually compile with these modifications), plus some notes in the README about change to needing to pass the current isolate to some V8 calls.

/cc @trevnorris