Update permalinks to jekyll's level

[laedit]

Permalinks should be the same as jekyll's: http://jekyllrb.com/docs/permalinks/

Now the default permalink will generate categories separated by / and not - like before, but a :dashcategories option can be used for those who prefer the -.

And the support of :category[n] (where n is an optional index of category) is still here.