Rets

* Search method now returns a list instead of an iterator. This allows us to catch maxrows internally and automatically

make subsequent reqeusts with offsets rather than forcing the client to catch and adapt. As the size of the reply is often
small, getting even ~32k listings of data was >300k in memory, this results should not affect the memory footprint.

* Added the `auto_offset` parameter for Session.search method. Defaults to True for making subsequent search requests

if the RETS server truncated the number of requested listings.

* Significatn changes to exception raising. No more InvalidFormat exception. ValueErrors are more appropriate for

input errors and RETSExceptions for consistently handling non-zero reply codes from the RETS Server. The RETSException
now has reply_code and reply_text parameters.

* get_object requests with location=1 now parse the response appropriately

* getObject dictionaries now include md5 fingerprints as content_md5

* Multipart image downloads working in Python2. Still not working in Python3

* No results continues generator