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.
I'm not sure I understand. If you're going to ship 2 libs, one without the jni symbol and 1 with.. why would one ever choose to use/deploy the one without? The size difference and added attack surface are practically zero, so I'm not understanding the benefit.

If _jni was shared and depended on the main lib, I'd agree. But I'm not following you in the static case.