change  `.jqtree_common` and some css 

Hi,
it looks like too deep cascading level in css, and i trying to fix it. 
so now it is more comfortable to modify style

* make low cascading level
  * remove dangerous element selector (previously, `.jqtree_common` was do it.)
  * remove unnecessary cascade level (like `ul.jqtree-tree`)
* change confusing `jqtree_common` (it using underscore naming & too abstract)

i'm sorry about my poor english :-(
check it out!