set `_in_db` flag to False after instance.destroy
fix sample link
stop using @property `count` for selected results
close cursor after execute sql
close cursor after execute sql
fix mysqldb
close connection on Database __del__
v0.7.1
upgrade to v0.7.1
Merge branch 'dev'

* dev:
  upgrade to v0.7.1
  v0.7.1
  close connection on Database __del__
  fix mysqldb
  close cursor after execute sql
  close cursor after execute sql
  stop using @property `count` for selected results
  fix sample link
  set `_in_db` flag to False after instance.destroy