Changes for version 0.24

• spelling fix [gregor herrmann]
• Added tests for extension markers (fails currently) [Niels Laukens]
• Bugfix for extension markerss in SETs and CHOICEs [Niels Laukens]
• Skip attempting to encode opEXTENSIONS inside of CHOICE
• Support for default explicit tagging [Peter Mogensen]
• Added tests and doc for explicit tagging [Peter Mogensen]
• Add new opEXPLICIT for sequences that are inserted for EXPLICIT tags