Fixed and improved wd plugin

I apparently made a booboo by somehow nesting the directories. This is now fixed.

Also took the opportunity to improve:

 * Added the the stuff I put in the wiki in a README file for good convention.
 * Added some checks, see [commit](https://github.com/mfaerevaag/wd/commit/f0ca48545bff4cae6a6198a4e244e748f303860f).
 * Added completion feature, see [commit](https://github.com/mfaerevaag/wd/commit/baabc69145c92f57c3ba614fdc2768415904d000).

Have squashed all commits to keep your commit log pretty.