Adding filters in backend Installation Discover view

+ It was very hard to find and install from discover view when having a large number of items. 
+ I have added filters in discover view like we have in other Joomla! view.
+ Here I have attached screen dump of the filters, 
![discover filters](https://f.cloud.github.com/assets/2002921/798629/dea6f01c-ed64-11e2-8770-2f7fa1259037.png)
+ **To test this PR**
   + Try to discover the items in installation/discover view.
   + Try to filter discover list with **any sting** and filter by **location**, **type** and **folder** using select box.