OpenCV 4.0 support

[fzyzcjy]

Hi! Thanks for this wonderful project! Now OpenCV4 has released, so how should I update it to V4? I am willing to give a PR!

[leadrien]

Hello,
Thanks for your proposal! Any help is always appreciated ;-)

[leadrien]

You can make a Pull request. Target branch Master

[fzyzcjy]

OK I will try to follow the steps :)

[fzyzcjy]

In addition to following the steps in README.md, I find several more things to notice:

1. change opencvlibrary/build.gradle res.srcDirs = ['/build/master_pack-android/opencv/modules/java/android_sdk/android_gradle_lib/res'] to `res.srcDirs = ['res']
2. in makefile, libopencv_java3 to ``libopencv_java4`.