Adds management of SVN externals through magit-svn-mode

This is roughly based on the post I made [here](https://groups.google.com/forum/?fromgroups=#!topic/magit/JW0MK16uSUg) a couple of months back.

Assuming the user has cloned all of their SVN externals into a single
directory which can be specified by magit-svn-externals-dir and then
symlinked in the appropriate places, "x" for "Fetch Externals" is added
to the menu and individually fetches and rebases each git svn repository
for the externals.