Added AutoSave feature as an enhancement (Issue 131)

Hi. This is my first contribution to Processing. I've done the following:
1) Modified the Editor.java file to call handleSave() in the prepareRun() function.
2) Modified defaults.txt to include an autosave.set option. The user can now set whether or not he wants to autosave through his preferences.txt file.

This commit closes [#131](https://github.com/processing/processing/issues/131)