Change signature for NativeHookException access

The jni_ThrowNativeHookException function tried to access a constructor
for NativeHookException with the signature (SLjava/lang/String;)V using
GetMethodID. This lead to a NoClassDefFoundError.
This fix changes the signature parameter to GetMethodID to
(ILjava/lang/String;)V for which a corresponding NativeHookException
contructor can be found.
Merge pull request #26 from tinarm/nhe-bad-signature

Change signature for NativeHookException access from short to int.  Thanks to tinarm for the patch.