Improve quoting in execute_with_dll() (xolox/vim-easytags#51)

I'm not yet 100% convinced this will work in all cases & environments,
because the rule* seems so crazy and arbitrary (coming from a UNIX
background). However I did test this quite a bit and it seems to work
reliably... Depending on how you read the documentation it could also
make sense :-)

For further discussion, see:
  https://github.com/xolox/vim-easytags/issues/51

* "The rule": If your command line is properly quoted, just wrap it in
  another set of double quotes without escaping any of the double quotes
  inside and it will Just Work... Who comes up with this stuff?!