simplify `return callback()` alternative

The value of `shell_cmd` is set in every of the above alternatives, and thus a simple `else` would suffice here.
node --eval "require('./')"

This change is intended to fix mapbox/node-pre-gyp#46, i.e. the module is required (not executed).

Some initial preparations are made for mapbox/node-pre-gyp#47, e.g. the node-webkit-specific check is copied to its own part of the code and removed from Node's.
create node-webkit-based module test's HTML file
create node-webkit-based module test's manifest file

The window is told to appear hidden.

In our application it remains hidden until `process.exit(0)` or `process.exit(1)` happens.
run node-webkit-based module test
bugfix: return after node-webkit-based module test is scheduled
do not become startled if node-webkit's stderr is not empty

Todd Wolfson at https://twolfson.com/2013-09-22-debugging-travis-ci says it's fine to ignore the `Xlib` error for `RANDR` even if it appears in the console.
Merge pull request #50 from Mithgol/node-webkit-based-module-validation

implement node-webkit-based module validation