Dbrequests

- dbrequests.mysql
- new function in database: send_delete for deleting rows in a database.
- send_data in mode 'update' now allows to send only partial subset of
columns.

- dbrequests.mysql
- see 35: New send_data modes: update_diffs, insert_diffs, replace_diffs

- dbrequests:
- see 27 for bugfix in Database class when specifiying a port in a
credentials object.
- the argument 'creds' in the init method of a database class is now
deprecated
- the argument 'db_url' can now handle str and dict type; str is a
sqlalchemy url; a dict a credentials object
- credential objects can now have additional fields which will be used as
elements in connect_args for sqlalchemies create_engine: see 12
- dbrequests.mysql
- see 36 for bugfix while sending an empty frame

- dbrequests.mysql:
- see 32 for bugfix in send_query for empty result sets

- dbrequests.mysql
- see 30: fixes handling of None/NULL values in columns

- dbrequests.mysql:
- see 22 for performance improvements for send_query when reading large datasets
- support for pymysql and mysqldb