Add Feature: Filter results by tag

1. add tag buttons on toolbar.
2. append count to tag button display name. 
3. click tag button to hide / show  todos of the tag
4. tag button is show if there has some todos of the tag, otherwise not. 

the screen shot looks like this:
![image](https://f.cloud.github.com/assets/1855738/1840423/37286626-7492-11e3-814b-46a3dbb53f2b.png)
![image](https://f.cloud.github.com/assets/1855738/1840444/73672ee2-7492-11e3-8227-13bd528f7cd8.png)

I hope you will enjoy this feature.

5. add chinese localization