[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/environment.md

@@ -133,17 +133,4 @@ the `QWeb` instance and a `browser` object:
 - `qweb` will be set to an empty `QWeb` instance. This is absolutely necessary
   for Owl to be able to render anything
 - `browser`: this is an object that contains some common access points to the
-  browser methods with a side effect. This is particularly useful when one want
-  to test more advanced components, and be able to mock those methods.
-
-More specifically, the `browser` object contains the following methods and objects:
-
-- `setTimeout`
-- `clearTimeout`
-- `setInterval`
-- `clearInterval`
-- `requestAnimationFrame`
-- `random`
-- `Date`
-- `fetch`
-- `localStorage`
+  browser methods with a side effect. See [browser](browser.md) for more information. Note that the browser object will be removed from the environment in Owl 2.0.