Have global exports be compatible with Web Workers

Assigning to `window` only works in a normal browser environment;
however, Web Workers don't have access to `window`. Instead, the global
object inside Web Workers is `self`.

Browserify ensures that `global` is present to all scripts by creating a
shim reference to it using this logic:

    typeof global !== "undefined" ? global :
    typeof self !== "undefined" ? self :
    typeof window !== "undefined" ? window : {}

Thus, if neither `global` nor `self` were originally present,
`global === window` will be true.

Web Workers compatibility was broken in a81e555.

support/browser-entry.js

@@ -157,5 +157,5 @@ Mocha.process = process;
  * Expose mocha.
  */
 
-window.Mocha = Mocha;
-window.mocha = mocha;
+global.Mocha = Mocha;
+global.mocha = mocha;