don't throw web audio api error on load

- since errors stop execution in a js file, if woscope was concatenated with other files for deployment, it could break an app. this is fixed now.
- error on woscope invocation instead
- send web audio API errors to error callback

.jshintrc

@@ -2,9 +2,5 @@
   "esnext": true,
   "browser": true,
   "browserify": true,
-  "devel": true,
-  "globals": {
-    "AudioContext": true,
-    "webkitAudioContext": true
-  }
+  "devel": true
 }

index.js

@@ -14,15 +14,6 @@ let shadersDict = {
 };
 
 let audioCtx;
-try {
-    try {
-        audioCtx = new AudioContext();
-    } catch(e) {
-        audioCtx = new webkitAudioContext();
-    }
-} catch(e) {
-    throw new Error('Web Audio API is not supported in this browser');
-}
 
 function axhr(url, callback, progress) {
     let request = new XMLHttpRequest();
@@ -39,6 +30,8 @@ function axhr(url, callback, progress) {
 
 module.exports = woscope;
 function woscope(config) {
+    audioCtx = audioCtx || initAudioCtx(config.error);
+
     let canvas = config.canvas,
         gl = initGl(canvas, config.error),
         audio = config.audio,
@@ -95,6 +88,19 @@ function woscope(config) {
     });
 }
 
+function initAudioCtx(errorCallback) {
+    try {
+        let AudioCtx = window.AudioContext || window.webkitAudioContext;
+        return new AudioCtx();
+    } catch(e) {
+        let message = 'Web Audio API is not supported in this browser';
+        if (errorCallback) {
+            errorCallback(message);
+        }
+        throw new Error(message);
+    }
+}
+
 function initGl(canvas, errorCallback) {
     let gl = canvas.getContext('webgl') || canvas.getContext('experimental-webgl');
     if (!gl) {