move next_token() into token.c
rewrote tokenizer to support the human readable strings that appear at
the end of some imap server responses

use callbacks to read/write data so that applciations can wrap the imap
library to support SSL, etc.

add callback for printing debug messages rather than print to stderr.

pass the human readable text in tagged server responeses to
imap_result_callback_t.

remove most of the extraneous token_type_* enum values and only support
the basic data types: string, number (except for ok, bye, no and bad).