Remove unnecessary call to require()
Fixed method call in dbIsValid
Revert "Fixed method call in dbIsValid"

This reverts commit 280c31e896490cc32d81eff5623840b82bda7932.
Fix method call in dbIsValid
Remove file which is duped in dplyr.R
Roxygenised
Plugging into dplyr interface (#4)

- Qualifying dplyr function names per new convention
- sql_select API changed: no "limit" parameter as this is unsupported by
SQL Server. Using top, offset & fetch.
- build_query coped to RSQLServer from dplyr and call to sql_server
reflects necessary API change mentioned above.
- add db_query_rows() method as SQLServer cannot ORDER BY in a subquery.
However, it does have the nice `@@ROWCOUNT`  value