Added Droplr Plugin

Thought CloudApp is pretty cool, I still prefer Droplr. Unfortunately, access to it's API is pretty much closed, so the workaround was to "remote control" the desktop client itself to do the job.
Hi @fabiofl, a couple of little fixes since this is taking a long time getting merged:

- The shebang here is not needed since plugin files are sourced, not executed.
- Both files don't end in newline (this is my OCD talking)
- In case this plugin is copied into the customs folder (because a user wants to modify it, for instance), the path to the ruby file will be `$ZSH_CUSTOM/plugins/...`. To support different paths, you can use the following construct:

```sh
DIR=$(dirname $0)
alias droplr=$DIR/droplr.rb
```

This is absolutely not necessary but it would be nice to have kind of a standard way of doing this; plugins `gitfast`, `catimg` and `omz-bootstrap` already do this.