allow people to set global default configs

Hi,

This branch adds a feature allowing people to set global default configs. 
For example:

~~~javascript
// file: app.js
ZeroClipboard.defaults.path = "http://kanbox.cn-hangzhou.oss.aliyun-inc.com/assets/zeroclipboard-b2be34bb5257ed269fbbbeabdd191b2f.swf"

// file: some-controller.js
$(btn).zclip({
  copy: 'my text'
})
~~~

I believe this can help people get rid of setting custom configs everywhere.

Cheers!