Add to_text method for Zone objects

When used as a library, it is handy to be able to get the Zone file's resulting text after making modifications to the zone.  Writing this out to a file is an unnecessary IO overhead when it can be written to a StringIO buffer and returned as a string instead.

I attempted to make this work in all currently supported Python versions (2.4+).  A test was written using the testFromFile1 test as a base.