Addressing comments from Tor and Sven.

- Added the method is_valid(not virtual though) to class Log_event
- Added the method get_type_code(Not virtual though) to class Log_event
- Removed the method get_event_name from libbinlogevent as it is not required there.
- Added the method get_type_str in the server code(in class Log_event).
- Moved the methods used for calculating binary_length of time, datetime, and timestamp
  from server to libbinlogevents.
- Code beautification.