[Bioperl-pipeline] Notice on the modification on XMLImporter

[Juguang XIAO]

For biopipe developers only.

I move some logics in 930-line Bio::Pipeline::XMLImporter into 
Bio::Pipeline::AbstractXMLImporter, which now is the parent module of 
XMLImporter. The method moves are

1. accessor methods for qw(dbhost dbname dbuser dbpass schema xml dba)

2. _prepare_dba

3. _prepare_xso

After the XMLImporter module is slimed, I do the same on 
XMLImporter::run methods to move some logic out into subroutine in 
XMLImporter, such as _parse_global, _parse_rules, _parse_jobs.

Juguang