added support for custom drawer indicator

I had to merge the branch from the other PR first (#28) because this requires to set up the ActionBarDrawerToggle in a separate function. It is used like this:

```javascript
TiDrawerLayout.createDrawer({
  leftView: menuView,
  centerView: contentView,
  drawerIndicatorImage: "/images/drawer_indicator.png",
  leftDrawerWidth: 260,
  width: Ti.UI.FILL,
  height: Ti.UI.FILL
});
```