Changes in samples build scripts

- Adding devmode task to all ant samples with the option to run
  the classic model adding gwt.args=-nosuperDevMode
- Detect whether the example is a maven build, and notice
  the user about it.
- Upgrade mobilewebapp gae version

Change-Id: I4061ea6d2d699bcb8ab437f1b60e3ff0c08cd3f2