Allow spaces in powerline fonts source dir

Fixes an error when running `install.sh` within a path containing spaces (and eventually other special characters), in my case `/Volumes/Big HD/Users/sebu/SCM/3rd-party/powerline-fonts--git`

I got the following error, which I fixed by this PR:

> find: /Volumes/Big: No such file or directory
> find: HD/Users/sebu/SCM/3rd-party/powerline-fonts--git: No such file or directory