Holland Plugin reorganization

This is a proposal to reorganize the holland/plugins/ directory. The idea here is to group multiple plugins under common namespaces with an eye towards their dependencies. mysql-lvm, for instance currently requires both holland.lib.mysql and holland.backup.mysqldump anyway, but here we can have a since parent holland-mysql package. The functionality in holland.lib.common should really be absorbed by the core holland package - this is some basic utility functionality and compression support. This is intended for some release beyond the 1.1 line.

This is how I envision the new plugins/ layout:

* plugins/lvm - effectively holland.lib.lvm and the generic lvm snapshot plugin framework. In the future a generic lvmsnapshot plugin can go here.

* plugins/mysql - core mysql plugins that only rely on commandline tools (essentially holland.lib.mysql + holland.backup.mysqldump). we may stuff additional plugins in here that only rely on the commandline tools at some point. xtrabackup may make sense here for now, as well. Delphini (the NDB cluster plugin) would go here here as well.

* plugins/mysql-lvm - mysql-lvm snapshot support. this would be both the archive-based and mysqldump ("mylvmdump") based plugins and depend on both holland-lvm and holland-mysql

* plugins/pgsql - similar to holland-mysql, but just relying on pg_dump and libpq (indirectly via psycopg2). If someone is so inclined, we can add pg_basebackup there as well.