Better Windows support (primarily handling paths with spaces in them)

This doesn't generally matter on Linux systems, but Windows machines are pretty likely to have paths with spaces in them. Quoting the paths prevents this from causing problems.
On Linux systems, if the path to the editor executable contains a space, eyaml edit currently fails if EDITOR does not contain quotes.

````
$ ln -s /usr/bin/vim /usr/bin/vi\ m
$ export EDITOR=/usr/bin/vi\ m
$ echo $EDITOR
/usr/bin/vi m
$ eyaml edit somefile.eyaml
[hiera-eyaml-core] !!! Editor did not exit successfully (exit code 127), aborting
[hiera-eyaml-core] ["/opt/puppet/lib/ruby/gems/1.9.1/gems/hiera-eyaml-2.0.3/lib/hiera/backend/eyaml/subcommands/edit.rb:50:in `execute'", "/opt/puppet/lib/ruby/gems/1.9.1/gems/hiera-eyaml-2.0.3/lib/hiera/backend/eyaml/CLI.rb:45:in `execute'", "/opt/puppet/lib/ruby/gems/1.9.1/gems/hiera-eyaml-2.0.3/bin/eyaml:13:in `<top (required)>'", "/usr/local/bin/eyaml:23:in `load'", "/usr/local/bin/eyaml:23:in `<main>'"]
$ export EDITOR=\"/usr/bin/vi m\"
$ echo $EDITOR
"/usr/bin/vi m"
$ eyaml edit somefile.eyaml # succeeds
````
I don't imagine that's a particularly common setup, but executing this block gets it to work with or without quotes in EDITOR.