Automatic inheritance (#29)

Corresponding wrapper trait automatically extends its Scaloid parent in
accordance with Android class hierarchy. If the parent wrapper not
exists, it raises compile error that helps avoid mistakes.

Related changes:

 - richClassDef/wholeClassDef template has optional mixin argument for
   manual inheritance.

 - Wrapped `android.preference.Preference` and
   `android.preference.DialogPreference` that are ancestors of
   `android.preference.EditTextPreference`.

 - Renamed `defaultValue` to `_defaultValue` due to confliction with
   `android.preference.DialogPreference`, and made it private in Scaloid
   packages.
renamed template argument 'cls' to 'base'
Removed debug message
Extract type variable with upper bounds
Extract generic class type
Automatic class hierarchy detecting
Merge pull request #31 from guersam/tmp/handle-abstract

Automatic class hierarchy handling