catch NoClassDefFoundError when scanning for model classes

The dex file format has a limit of 65k method definitions. [It's easy to run into that limit](http://jakewharton.com/play-services-is-a-monolith/) when using common android libraries. One workaround is to [strip unused packages out of downloaded libraries when building](https://gist.github.com/dmarcato/d7c91b94214acd936e42). 

Unfortunately, if you do this, when ActiveAndroid initializes, it throws `NoClassDefFoundError`s for those packages when scanning for model classes. Here's a sample:

```
E/AndroidRuntime(28605): java.lang.NoClassDefFoundError: com.google.android.gms.internal.jf$1
E/AndroidRuntime(28605): 	at java.lang.Class.classForName(Native Method)
E/AndroidRuntime(28605): 	at java.lang.Class.forName(Class.java:305)
E/AndroidRuntime(28605): 	at com.activeandroid.ModelInfo.scanForModelClasses(Unknown Source)
E/AndroidRuntime(28605): 	at com.activeandroid.ModelInfo.scanForModel(Unknown Source)
E/AndroidRuntime(28605): 	at com.activeandroid.ModelInfo.<init>(Unknown Source)
E/AndroidRuntime(28605): 	at com.activeandroid.Cache.initialize(Unknown Source)
E/AndroidRuntime(28605): 	at com.activeandroid.ActiveAndroid.initialize(Unknown Source)
E/AndroidRuntime(28605): 	at com.activeandroid.ActiveAndroid.initialize(Unknown Source)
E/AndroidRuntime(28605): 	at com.activeandroid.ActiveAndroid.initialize(Unknown Source)
```