Add support for package maintainer scripts.

Debian allows packages to have {pre|post} install and {pre|post} removal
scripts. They are used to manage and configure the system in ways that files
alone cannot (e.g. add user) This change allows users of the sbt native
packager to include these files (optionally) in their generated debs.
Merge pull request #10 from saeta/saeta-debian-maintainer-scripts

Add support for package maintainer scripts.