Attempt to solve snow clash when snow protected page opens itself

[weizman]

Attempt to fix #81 (after ditching maybe a too complex idea on first attempt @ #82)

• Change Snow so that for each intercepted window it sets the SNOW property to the new window.
• Change package.json to add a line at the very beginning of Snow execution so that if prop SNOW already exists this means parent already applied Snow here and we don't have to continue.
• This helps specifically in case the new realm loads the same page which loads a 2nd Snow instance. This tells the 2nd instance to bail and trust the parent instance.

[weizman]

Seems that this solves a scenario where Snow code is loaded again in a same origin iframe, but doesn't cover a scenario where that instance of Snow is actually being called. fixed at #121