Add MatrixClient.initCrypto

initialising the crypto layer needs to become asynchronous. Rather than making
`sdk.createClient` asynchronous, which would break every single app in the
world, add `initCrypto`, which will only break those attempting to do e2e (and
in a way which will fall back to only supporting unencrypted events).

spec/integ/matrix-client-methods.spec.js

@@ -305,6 +305,10 @@ describe("MatrixClient", function() {
             return;
         }
 
+        beforeEach(function() {
+            return client.initCrypto();
+        });
+
         it("should do an HTTP request and then store the keys", function(done) {
             const ed25519key = "7wG2lzAqbjcyEkOP7O4gU7ItYcn+chKzh5sT/5r2l78";
             // ed25519key = client.getDeviceEd25519Key();