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.
Well, in terms of expected usage, I think you either want the library itself (which offers the API described in the .h file), or you want something to plug into Java (which offers the API described by the .java file). I wasn't expecting one library that could function as both, but in practice there's probably very little benefit in splitting it up.

One alternative could be to have a different top-level .c file which implements the Java API, which may be different (and for now, would definitely just be a subset), but that's probably overkill as well.

Concept ACK.