full support mzid 1.0 and 1.1

Summary of changes:
- column names in all mzID slot data.frames (except database) have been set to lower case
- `object@evidence@evidence` has new column `peptide_ref`, created from `object@evidence@evidence$id`.
- fixed issue #8

Identity has been tested with loaded two identical searches exported to v1.0 and v1.1.