added new defined types and changed readme

ohmyzsh::theme { "user": theme => "theme" }
ohmyzsh::plugins { "user": plugins => "plugin1 plugin2 ..." }

both require ohmyzsh::install

also added a check to ::install if the user already is defined.
if so, the resouce is changed to set zsh as shell
Merge pull request #1 from lotherk/lotherk

added new defined types and changed readme