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.java

@@ -0,0 +1,35 @@
+package org.bitcoin;
+
+/**
+ * This class holds the context reference used in native methods 
+   to handle ECDSA operations.
+ */
+public class Secp256k1Context {
+  private static final boolean enabled; //true if the library is loaded
+  private static final long context; //ref to pointer to context obj
+
+  static { //static initializer
+      boolean isEnabled = true;
+      long contextRef = -1;
+      try {
+          System.loadLibrary("secp256k1");
+          contextRef = secp256k1_init_context();
+      } catch (UnsatisfiedLinkError e) {
+          System.out.println("UnsatisfiedLinkError: " + e.toString());
+          isEnabled = false;
+      }
+      enabled = isEnabled;
+      context = contextRef;
+  }
+
+  public static boolean isEnabled() {
+     return enabled;
+  }
+
+  public static long getContext() {
+     if(!enabled) return -1; //sanity check
+     return context;
+  }
+
+  private static native long secp256k1_init_context();
+}