Introduce a BaseActivity

* onDestroy() and unbindDrawables() now in BaseActivity where possible
* onOptionsItemSelected() for android.R.id.home now in BaseActivity

I changed the unbindDrawables() call to be more generic.
Source: http://stackoverflow.com/a/5069354

"Proof" that it's the same: LogCat output once in the original **ACTI**vity
and once in the **BASE** Activity.

```
08-14 14:05:25.023: I/BASE(11457): android.widget.RelativeLayout{41f07968 V.E..... ........ 0,0-768,1038  #7f07003f app:id/ProjectActivityRoot}
08-14 14:05:25.023: I/ACTI(11457): android.widget.RelativeLayout{41f07968 V.E..... ........ 0,0-768,1038  #7f07003f app:id/ProjectActivityRoot}
```

```
08-14 14:05:31.770: I/BASE(11457): android.widget.RelativeLayout{41f8b148 V.E..... ........ 0,0-768,1038 #7f07002e app:id/MyProjectsActivityRoot}
08-14 14:05:31.770: I/ACTI(11457): android.widget.RelativeLayout{41f8b148 V.E..... ........ 0,0-768,1038 #7f07002e app:id/MyProjectsActivityRoot}
```

```
08-14 14:05:35.494: I/BASE(11457): android.widget.LinearLayout{41dc6670 V.E..... ........ 0,0-768,1038 #7f070027 app:id/main_menu}
08-14 14:05:35.494: I/ACTI(11457): android.widget.LinearLayout{41dc6670 V.E..... ........ 0,0-768,1038 #7f070027 app:id/main_menu}
```

This change is up for discussion. If it's welcome, if we don't want it etc.
@robertpainsi @kaufi @sjaindl @felly What do you think?