PLOS Search ported to Spice2

Updated version of PLOS Search plugin originally submitted in #153 (see link for description).

No major changes in the code. Just did a cleanup, removed css file since it was not being used, and reduced the handlebars template to a function call that builds the reference list. Let me know if it is better to iterate within the template (I would think that it would be slower then processing everything in the .js).

Also, I have removed the top attribution line _Data Provided by PLOS_ as discussed, so the latest snapshot looks like this:

![screenshot from 2013-10-19 11 25 05](https://f.cloud.github.com/assets/361179/1365957/d9a5e866-38a0-11e3-9be3-ea65d8e70134.png)

However, I have contacted PLOS through their [API developer's group](https://groups.google.com/d/msg/plos-api-developers/lpLymk9afPY/-QE2MLEUAvEJ) so that they can give an official position about the display policy.

Other than that I need to fill in the metadata in the plugin file, but I didn't find the [Metadata README](https://github.com/duckduckgo/zeroclickinfo-spice/blob/master/metadata.md) file mentioned on https://github.com/duckduckgo/zeroclickinfo-spice/blob/master/CONTRIBUTING.md