Signing RPMs via Bouncy Castle Security Provider

Actually, this is a first shot to implement RSA signing for RPMs (also mentioned in #14). I'm not an expert in JCE, therefore I used Bouncy Castle PGP directly. I'm sure there will be a smart guy that can easily port the stuff to JCE afterwards.

I verified the implementation via ```rpm -K <rpm-file``` on Scientific Linux 6 and Ubuntu 12.04. On Scientific Linux you could verify the test result by doing the following:
```
$ git clone https://github.com/craigwblake/redline.git
$ cd redline
$ mvn test
$ sudo mkdir -p /var/lib/rpm/pubkeys/
$ sudo cp src/test/resources/pgp/public.key /var/lib/rpm/pubkeys/
$ rpm -vK target/signing-test-1.0-1.noarch.rpm
```