change typeof <v> !== 'undefined' to <v> !== void 0

This request is to modify the places in the code where "typeof <v> !== 'undefined'" to "<v> !== void 0" where <v> is a variable (in this case exports and window).

A few reasons for the request:
1)  I have seen performance improvements by going with the void 0 approach.  Supporting data: http://jsperf.com/typeof-vs-undefined-check/15
2)  It reduces the overall number of characters used to test whether the variable is undefined (an overall bandwidth improvement).
3)  Although 1 & 2 are negligible for this project due to only being used in a few places, this seems to be a best practice being used by other libraries (i.e. underscore.js) and would be good to use for continuity of best practices between projects.  

Thanks!