jnius: refactor to not save jni env, and use the current pushed/pop env. This resolve few issues when pyjnius is called from a proxy (error such as E/dalvikvm( 6036): JNI ERROR: env->self != thread-self (0x68415860 vs. 0x400bb010); auto-correcting)
Merge branch 'master' of github.com:kivy/pyjnius