1. updating TODO list, bumping up version to b3

2. solved issues reported in github, including
  2a assertation error for asn>0 while conevrting certain files
  2b improved function for downloading latest (contributed)
  2c minor update to documentation
  2d fixed get_as_prefixes for non-int parameters

3. added pyasn.__version__

4. added caching to get_as_prefixes; added unit-tests