added code of official libwebp 0.4.2

- from: https://chromium.googlesource.com/webm/libwebp/
removed unused .gitignore
removed encoder bits

- also removed everything we don't need from libwebp.swig (go, python) and removed all encoder bits from there, too
added auto-generated code from swig

- command: swig -java -package com.google.webp -outdir java/com/google/webp -o libwebp_java_wrap.c libwebp.swig
- moved Java files to their destined place
added Application.mk, added 'out' folder to gitignore
disabled executables (examples) and enabled shared library by default
added compiled libs from 'ndk-build' (and moved them to the correct 'jniLibs' directory)
refactoring libwebp API
added WebPBackport and instrumentation test

- also added example WebP files
- added x86 library (for instrumentation tests only) to be able to run tests in the fast emulators, for extended developer happiness
enabled test app to load WebP
moved module libwebp to webp_backport