Replace Python string formatting with calls to native code

Make the Python entry.to_json() and entry.to_text() methods thin
wrappers for dnstable_entry_to_json() and dnstable_entry_to_text().

In a libdnstable with the new dnstable_entry_set_iszone() function, the
native text and JSON format functions produce equivalent output to what
we were previously producing with the Python code, and the native
functions are much faster, of course.
Merge pull request #1 from farsightsec/branches/native-text-and-json

Replace Python string formatting with calls to native code