JNI FindClass result caching and AttachCurrentThread if it was detached.

Caching results of FindClass calls (wich are used a lot, at least once per frame) for better perfomance, a "best practice" as explained here: http://www.ibm.com/developerworks/library/j-jni/#notc .
Call to AttachCurrentThread if it was detached, should be possible now to do calls to JNI code from different natives haxe threads.