Fixed add in zsh <= 4.3.10 & Updated error messages

Zsh bug means that using =~ and || in tandem is not safe, so the
warp point verification checks are all done in separate if statements
now. This also allows us to provide more precise error messages for
why a warp point name has been rejected.

See http://www.zsh.org/mla/workers/2014/msg00501.html for zsh bug
details.

At time of writing, some RedHat and CentOS still have 4.3.10 as
the most up-to-date zsh in their package managers, so this should
allow wd to work on those systems properly.
Merge pull request #6 from hamiltont/patch-1

Fixed bug in add on zsh <= 4.3.10