Merge pull request #15 from cuviper/master

Map <Leader>- similar to :Sexplore
Fix jumps to the root directory.

It seems that sometimes netrw loses the current dir name and
expand('%') becomes empty string.
Using '-' in this situation will jump to the root directory.
This fix saves the original netrw's '-' mapping and uses it to
browse up through folders.
Merge pull request #16 from serebrov/master

Fix jumps to the root directory.