Conditional exports assignment.

exports / module.exports are undefined browser-side (as suggested in the comment on line 15) is used only server-side, 
so it breaks browser-side implementations to assign them without checking if they exist first.
Merge pull request #30 from FreakenK/master

Fixing browser-side error 'module is not defined'
bug fix for AMD, checked `window` element instead `require`.
Updated performance comparison section
Merge branch 'master' into gh-pages