…up on Android 4.x

- Fixed "_build.js" to create main dex class list if the min API Level is less than 21, not the target API Level.
  * Needed since Android 4.x does not support multidexed apps.
- Modified main dex class list to not include external module classes.
  * Significanlty reduces classes added to main dex list. Needed for anything using Google Play Services (adds too many APIs).
  * Modified app class template to init modules via reflection instead since module classes might not be in main dex file.

- Added Android "gradle" build template.
  * Currently used to acquire/run newest ProGuard version. (Android SDK's ProGuard is old and doesn't support Java 8.)
  * Not used to build Android apps, yet.
- Added new "proguard.gradle" script.
  * Downloads/acquires newest ProGuard version via maven.
  * Can be used run ProGuard via gradle instead of via "java" command and the ProGuard JAR.
  * Accepts multiple ProGuard configuration files. (JAR version does not support this.)
- Fixed multidex ProGuard usage to work on Windows.
  * Was using a ':' colon as a file/path array separator instead of a ';' semicolon.