Export zxcvbn function conditional on the environment

Rather than directly attaching the zxcvbn function to the `window` object,
check first if the window exists (i.e., we are in a browser environment).

If not, then we may be in a CommonJS environment, and we can export the
function by attaching it to the `exports` object.

init.coffee

@@ -42,7 +42,7 @@ KEYPAD_STARTING_POSITIONS   = (k for k,v of keypad).length
 time = -> (new Date()).getTime()
 
 # now that frequency lists are loaded, replace zxcvbn stub function.
-window.zxcvbn = (password, user_inputs) ->
+zxcvbn = (password, user_inputs) ->
   start = time()
   if user_inputs?
     for i in [0...user_inputs.length]
@@ -54,4 +54,10 @@ window.zxcvbn = (password, user_inputs) ->
   result.calc_time = time() - start
   result
 
-window.zxcvbn_load_hook?() # run load hook from user, if defined
+# make zxcvbn function globally available
+# via window or exports object, depending on the environment
+if window?
+  window.zxcvbn = zxcvbn
+  window.zxcvbn_load_hook?() # run load hook from user, if defined
+else if exports?
+  exports.zxcvbn = zxcvbn