Incremental fixes and enhancements

Changes include:

• Add an optional alt argument to wrapAtom #12.
• Add an optional alt_type argument to wrapAtom #12.
• Don't write empty and tags in QueueXML. #9.
• Use xs:dateTime compatible date strings for lastChecked elements in bag xml. #7.
• Update requirements for simpler environment prep.
• Better error handling for ANVL parsing #3
• Validation for PREMIS Event XML tests