On bufleave we save the nerdtree window state. However, sometimes when
bufleave is triggered the tree is no longer open in this tab - e.g. for
secondary trees or mappings that open in another tab. Dont bother saving
screen state in these cases as the code doing the saving assumes the
tree is available in this tab.
Add 2 new classes and move code into them from autoload:
* NERDTree. Each nerdtree buffer now has a NERDTree object that holds
the root node and will old other util functions
* UI. Each NERDTree object holds a UI object which is responsible for
rendering, getting the current node, etc
Still a fair few methods to sort through in autoload (many of which will
end up in the above classes) - need sleep though.
