JNI library

Squashed and rebased. Thanks to @theuni and @faizkhan00 for doing
the majority of work here! Also thanks to @btchip for help with debugging
and review.

src/java/org_bitcoin_Secp256k1Context.c

@@ -0,0 +1,14 @@
+#include <stdlib.h>
+#include "org_bitcoin_Secp256k1Context.h"
+#include "include/secp256k1.h"
+
+SECP256K1_API jlong JNICALL Java_org_bitcoin_Secp256k1Context_secp256k1_1init_1context
+  (JNIEnv* env, jclass classObject)
+{
+  secp256k1_context *ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);
+
+  (void)classObject;(void)env;
+
+  return (jlong)ctx;
+}
+