add onlineDictionary extension,support dictionary.com etc

This extension will search the selected text with you choosed online dictionary service, now it supported  `dictionary.com`,`dict.cn` and `dict.youdao.com`. 

It will open a new tab in your default browser.


#### icon
 ![image](https://f.cloud.github.com/assets/681150/403082/4dea8cd2-a912-11e2-9076-8cec056c45ef.png)


#### option
* ![image](https://f.cloud.github.com/assets/681150/403078/171a5a84-a912-11e2-8856-bfc3312a0e9f.png)