new features

Since I started to use netrw I found a couple of annoying bugs, so I looked for an alternative. I wanted the file explorer to open in the current buffer, and nerdtree seemed not capable of doing it, so easytree looked like the almost perfect solution. The fact it was so small and well written allowed me to add what I was feeling it was missing. I have no idea if you still use / maintain your project, but I thought that merging back my modification could be nicer than having one more fork floating around, so I made this pull request.
I'm no vim expert, so feel free to show me errors or improvements to my code. I tried to maintain backward compatibility, so the new features need to be enabled explicitly. They are:

* use v instead of vs
* use s instead of sp
* use t to open a file in a new tab
* use space to fold/unfold directories (along with the old keymaps)
* maintain unfolded status between invocations without saving the settings to disk
* save all the settings under the same dir instead of scattering them around the disk

I've updated the docs as well.