Handle newly created tab separately in 'BufWinEnter' event

The 'TabEnter' event is triggered in the middle of establishing the new
buffer, it is not a good time to restore focus to NERDTree. At the time
when 'BufWinEnter' event gets triggered, the new buffer has been
successfully loaded, then we can restore the focus to NERDTree.
Make sure a NERDTree window gets opened in the new tab

Remove 's:MirrorIfGloballyActive()', use 's:NERDTreeMirrorOrCreate' instead.
Merge pull request #46 from techlivezheng/feature/refactor-open-on-new-tab

Refactor NERDTree opening mechanism in the new tab