Browserify compatibility

oauth-signature-js checks if a "window" global object exists to determine if it is running in the browser. While this works alone, it does not work when in conjunction with Browserify. 

Browserify emulates Node's "module" global to properly expose objects. In order to work with Browserify (or Node "emulation") I modified the isNode flag to check for the non-existence of the module and module.exports global, instead of checking for the existence of the window global. 

I don't know of a simple way to test that the code works with browserify and the current code does not. I added a test to prove that the "old" and "new" method of detection both return the same value when tested in the browser or in Node. Thus, no current implementations should be hindered by the update.