Add `saveScreenshot` API which actually saves to the filesystem.

The `takeScreenshot` API just returns base64 encoded data of the screenshot.
This method is more user-friendly and actually saves the image to the filesystem.

Usage:

browser
  .elementById("login")
  .click()
  .saveScreenshot("Logged_In")
  .elementById("logout" )
  .click()
  .saveScreenshot("Logged_Out");
added global http config
README update
deprecated setHttpTimeout
README update
doc update
unit test cleanup
README update
mapping update
implemented addAsyncMethod, addPromisedMethod, removeMethod + deprecator
added noop
added noop/sleep/chain
added resolve
adding doc
made sleep and noop element chainable
test fix
test fix
test fix
added support for alternate promise libraries
add extra methods update