expose core randomness

```javascript
freedom['core.crypto']().getRandomBytes(n).then(function (buffer) {
  ...
});
```
address freedomjs/freedom-for-firefox#33

interface/core.crypto.json

@@ -0,0 +1,6 @@
+{
+  "name": "core.crypto",
+  "api": {
+    "getRandomBytes": {"type": "method", "value": ["number"], "ret": "buffer"}
+  }
+}

providers/core/core.crypto.js

@@ -0,0 +1,31 @@
+/*globals console */
+/*jslint indent:2, node:true */
+var util = require('../../src/util');
+
+/**
+ * A Core provider for getting cryptographically random buffers. This
+ * functionality may not exist in all unpriviledged contexts - namely at this
+ * point, firefox addon workers.
+ * @Class Core_crypto
+ * @constructor
+ * @param {module:Module} cap The module creating this provider.
+ */
+var Core_crypto = function(cap, dispatchEvent) {
+  this.dispatchEvent = dispatchEvent;
+  util.handleEvents(this);
+};
+
+/**
+ * Get a random buffer of some number of bytes.
+ * @param {String} str The string to send.
+ * @param {Function} continuation Function to call when sending is complete.
+ * @method send
+ */
+ Core_crypto.prototype.getRandomBytes = function(number, continuation) {
+   var buffer = new Uint8Array(number);
+   crypto.getRandomValues(buffer);
+   continuation(buffer.buffer);
+};
+
+exports.provider = Core_crypto;
+exports.name = "core.crypto";

spec/providers/core/core.crypto.unit.spec.js

@@ -0,0 +1,25 @@
+var Crypt = require('../../../providers/core/core.crypto');
+
+describe("providers/core/Core_crypto", function() {
+  var crypt;
+
+  beforeEach(function() {
+    crypt = new Crypt.provider();
+  });
+
+  it("Generates random buffers!", function(done) {
+    crypt.getRandomBytes(50, function(buffer) {
+      expect(buffer).toBeDefined();
+      var view = new Uint8Array(buffer);
+      expect(view.length).toEqual(50);
+      var max = 0;
+      for (var i = 0; i < view.length; i += 1) {
+        if (view[i] > max) {
+          max = view[i];
+        }
+      }
+      expect(max).toBeGreaterThan(0);
+      done();
+    });
+  });
+});