Make Olm work with browserified js-sdk

We want to avoid distributing olm as part of the js-sdk, so we exclude it from
the browserified build. Previously this meant that you couldn't use olm this
way.

We can do better though: if the web page includes olm.js via a separate script
tag, that will set global.Olm, which we can get browserify to shim in.

src/crypto/OlmDevice.js

@@ -21,6 +21,11 @@ limitations under the License.
  * @module crypto/OlmDevice
  */
 const Olm = require("olm");
+if (!Olm) {
+    // this happens if we were loaded via browserify and the Olm module was not
+    // loaded.
+    throw new Error("Olm is not defined");
+}
 const utils = require("../utils");