Display the main document as a script file.

Fixed bugs related to main document implementation (internal file node.js):
- implemented PageAgent.getResourceContent() so that front-end displays
  file's content
- overriden FramePayload handling so that the main file is treated
  as a Script instead of a Document.
Merge pull request #5 from strongloop/main-script-file-fix

Display the main document as a script file.