Show a file that is currently opened.

This commit does the following:
* It adds a function `NERDTreeTabsFind` to show currently opened file in NERDTree,
* It adds a variable `g:nerdtree_tabs_autofind` to automatically show file while reading new buffers.

Also, it might be a solution to issue #30 
The spaces are there on purpose, they create a line break (http://daringfireball.net/projects/markdown/syntax#p). I'm going to re-add these, it seems to me the listing is a bit easier to read with the line breaks.