Hstore fields
This patch allows for specifying customfields in BugTrackers that can be mapped to existing Bug fields, or non-builtin fields. When Bugs get polled the fields defined in the BugTracker will be polled and populated.
Custom fields that map to existing fields, e.g. severity, will populate those fields. If a non-builtin field is specified, e.g. build_number, these fields will be populated in the Bug's custom_fields field.