Jni included in repository => user does not need a JDK for setup

The included jni headers have been taken from GNU classpath and are GPLv3, so are compatible with Apache license. The major advantage of including the headers is, that we do not require a JDK to be installed only for jni headers. 

First we check JAVA_HOME to include jni.h, and if it fails we fall back to provided jni.h

So far I've tested with travis and locally under java8 64 bit Ubuntu trusty with gcc-4.9 and clang++-3.3 and no errors so far. Would somebody please investigate under Windows and MacOSX?

Best,
Martin