Fixed DNSStrField.i2m() by truncating chunks of more than 63 bytes because they cannot be encoded (ticket #42)

This automaton accepts any TFTP WRQ request and returns the tranfered file

Most of the initialization is done once for all at the class creation instead of being done at each instantiation
Now the .graph() method can become a class method and can be called directly on a class

Renamed Automaton.cond and recvcond into conditions and recv_conditions to reduce risk of name collision

If "string" parameter is not None, the dot string is returned
"type" parameter is deprecated and replaced by "format" parameter
"options" parameter is added