Merge pull request #12 from koreader/lost-head

get dimension of default screen via FFI
Release UTF8 strings that the JVM gave us

From @thotypous, see https://github.com/koreader/android-luajit-launcher/issues/13
Merge pull request #14 from hwhw/jni-string-fix

Release UTF8 strings that the JVM gave us
use absolute path in FFI dlopen
This should fix #10.