Add include file option -i

This change adds the possibility to add additional files (like conf or service files) into the aurball via the mkaurball command line.

Example:
```
$> mkaurball -i selfspy.conf -i selfspy@.service
==> Making package: selfspy-git 0.1.4.r169.g363aa5f-2 (Fri Oct 24 02:46:00 CEST 2014)
....... [etc]
==> Leaving fakeroot environment.
==> Source package created: selfspy-git (Fri Oct 24 02:46:00 CEST 2014)

$> tar tf selfspy-git-0.1.4.r169.g363aa5f-2.src.tar.gz
selfspy-git/
selfspy-git/selfspy@.service
selfspy-git/selfspy.conf
selfspy-git/.AURINFO
selfspy-git/.Changelog
selfspy-git/PKGBUILD
selfspy-git/selfspy.install
```