Make functioning build with 'gulp dist'

[oddgoo]

'gulp dist' was not building a functioning distribution. System.js wasn't found and there was no executable entry point to the rest of the app.

To fix this, I:

• Changed the order of the htmlReplace js parameters, to put 'js/system.js' at the top.
• Added a new htmlReplace block, to add an entry point to 'src/app.js' in index.html

Perhaps this is better solved by flagging the app.js file as a --script, instead of a --module, but I did not know how to do that.