Compile Extension Fields to correct message class

The libprotoc Generator provides extension fields
to a FileDescriptor or Descriptor object. I
misunderstood that the descriptor extension fields
are not the fields that belong to the corresponding
message proto/class.

The fix is to store the extension fields by their
containing type name when we initially parse
through the messages in the file descriptor.
Then when we  go to print each message's fields
we print out the previously extracted
extension fields.

Fixes #69.
Merge pull request #75 from localshred/stable_fix_extensions

Compile Extension Fields to correct message class