[FIX/IMP] export browser object, prevent crash in private mode

This commit performs two tasks:

1. it exports the internal browser object (in owl.browser), for ease of
testing
2. it makes sure that Owl does not crash inside iframes in incognito
mode, because window.localStorage raises an exception in that case

closes #791

doc/reference/animations.md

@@ -52,7 +52,7 @@ sequence of events will happen:
 At node insertion:
 
 - the css classes `name-enter` and `name-enter-active` will be added directly
-  when the node is inserted into the DOM. 
+  when the node is inserted into the DOM.
 - on the next animation frame: the css class `name-enter` will be removed and the
   class `name-enter-to` will be added (so they can be used to trigger css
   transition effects).
@@ -115,11 +115,13 @@ If you use SCSS, you can use mixins to make generic animations. Here is an exemp
 ```
 
 Usage:
+
 ```scss
 @include animation-fade(0.5s, "o_notification");
 ```
 
-You can now have in your template: 
+You can now have in your template:
+
 ```xml
 <SomeTag  t-transition="o_notification_fade"/>
 ```