Conform to QGIS 2.0 plugin structure

Should close https://github.com/cfarmer/cartogram-plugin/issues/1.

I have followed the instructions [here](http://www.qgis.org/en/docs/pyqgis_developer_cookbook/plugins.html), and after a long trial-and-error process I have modified `doCartogram.py` in order to make it work again.

I have also conformed the structure of the plugin to the requirements for QGIS 2.0+ plugins and added a `metadata.txt` file. I retrieved and used some files from the previous [repo on Google code](http://code.google.com/p/ftools-qgis/source/checkout).

It is not perfect: somehow the attributes are listed two times in the Dialog and it can not use the data coming from a join (which, interestingly enough, was what I needed to do).

One of the issues is related to `PyQt.QtCore.QString` which I was unable to use successfully and to the fact that now some functions like `absoluteFilePath` return a `unicode` string instead of `PyQt.QtCore.QString` with much disgrace of the methods like `left()`, `right()` and so on.

Also, I suggest to change the name of the folder/repo in `cartogram_creator` because I encountered an issue which looks very similar to [this bug](http://hub.qgis.org/issues/3145) (maybe this is a case of regression?), I didn't double check, though.
Cristian