1.2.0: MODE_FILE fix when reader is loaded before forking

• Previously if MODE_FILE was used and the database was loaded before
  forking, the parent and children would use the same file table entry without
  locking causing errors reading the database due to the offset being changed
  by other processes. In MODE_FILE, the reader will now use os.pread
  when available and a lock when os.pread is not available (e.g., Python
  2). If you are using MODE_FILE on a Python without os.pread, it is
  recommended that you open the database after forking to reduce resource
  contention.
• The Metadata class now overloads __repr__ to provide a useful
  representation of the contents when debugging.
• An InvalidDatabaseError will now be thrown if the data type read from
  the database is invalid. Previously a KeyError was thrown.