enable libsecp256k1_jni shared lib

This enables a libsecp256k1_jni shared lib to be built. It depends on libsecp256k1.

```./configure --enable-jni``` should be all it takes assuming your java environment is setup properly. For now, there's no way to specify where the includes should be found if they're not located in $JAVA_HOME/include, as I'm not sure if that's necessary or not.

Notice that the lib has been renamed in the .java to reflect the lib's new name. This was done to match standard distro packaging names.

This is untested in a real java/jni build, verification that it actually works will be needed before merging.
No, why does libsecp256k1.la depend on the jni library? I would expect the other way around?