Adding *_using_(db) methods for class methods

As discussed earlier, if a model extends QueryExecutor then all the class methods will be extended with a _using_(db) method.  The db is assumed to be a sqlite db and line 36 of the QueryExecutor module appends a .sqlite3 extension; if moving to MySQL or Postgres then remove the extension from that line.  Informally tested in console with following cases:
```ruby
# the original class method
VisitAction.pageviews('AppBuilder') => 3; 
# I modified the log.sqlite3 db so one record matches
VisitAction.pageviews_using_log('AppBuilder') => 1;  
# relog.sqlite3 db doesn't exist, so we should raise an exception...
VisitAction.pageviews_using_relog('AppBuilder') => Exceptions::QueryNotExecuted   
```
More formal testing is obviously required.  Contact me if you have more questions about this -- I did add a bunch of comments to help, but metaprogramming and method_missing is not something we covered in 67-272 so I'd understand if you have questions.  Nice thing is that other models can also use this module and this technique very easily.