Implements from_text for the TSIG record type, and clean up some other
things.

Fixes the text format to emit fields in the right order; fudge and
time_signed were reversed.  This also matches BIND's output format now.

Add get_uint48() to the tokenizer, so that from_text() can use it.  Add
get_uint48() to the wire parser, and use it in from_wire, for
consistency.

Change dns.tsig.sign() to use rdata.replace() rather than constructing a
new TSIG record manually; this couldn't be done before, because
replace() uses text format for validation.

Replace identical code duplicated in a number of rdatatype
implementations that concatenated all of the remaining tokens in order
to decode them.

Add missing base parameter to get_uint32.

Don't copy get_int in get_uint32, just call get_int.

get_uint32 erroneously permitted 4294967296.

remove cut-and-paste origin parameter to get_string() and get_identifier().

Classes inherit from object by default; there's no need to explicitly
include this.

Replace super(Foo, self) with super().