Added BDD feature management / Basic PDF export

Hi There,

First thanks for your great work. It helped me to increase the productivity of my customer and to tighten the requirements definition process and management as well. Basically what the KOREM plugin was intended for.

As I suggested my customer to select a Behavior Driven Development (BDD) approach for their projects to collect requirements I added the possibility to KOREM to add so called *features* from the BDD philosophy to the Redmine database. For further information on this please read on {here}[http://pythonhosted.org/behave/philosophy.html].

One missing feature also was that it is not simply possible to export the requirements and other added data from the plugin. We could have done this via an external interface (e.g. database admin interface), but a direct export was mandatory.

So I spend the effort and also added the possibility to export the tree structure as flat PDF file by right clicking on the tree root node and select the "Export" option. This feature uses pdfkit which requires wkhtmltopdf - I updated the gem file accordingly and added some information to the README.rdoc file.

I would be happy if you merge the changes in your main stream and if we could get together to talk about further adjustments which I have in mind, not only related to the export function, but also to other missing features.

Cheers,
Marvin Kruse
Philotech GmbH