Release 0.2.3

This is a performance improvement release:

• BitString type implementation rebased onto Python int to behave like an integer and improve serialization performance (100x times!).
• Integer codecs reimplemented to base on Python's built-in integer serialization codecs to improve serialization performance (~40%).
• Decoding performance of Set and Choice types improved by caching once computed tags maps.
• ASN.1 character types refactored to duck-type unicode strings thus making them more convenient in text processing context.
• Documentation updated at many places.

More changes noted in the CHANGELOG.