implement screenshots

[matthewmueller]

fixed up the nightmare.screenshot(path) implementation. Basically passes the image buffer back from the electron process to the node process to write the file.

Here's an example:

var Nightmare = require('./')

Nightmare()
  .goto('https://google.com')
  .screenshot('screenshot.png')
  .end(function(err) {
    if (err) throw err
  })

[reinpk]

bloody beautiful :)

[matthewmueller]

we can probably encode differently if it's .jpg vs .png

[reinpk]

ah, hmmm let's just document "only png" in the readme for now and see if it's an issue.