Use .jcall rather than $ operator.

The former is faster and is recommended by package developer
Updated getVersion return type
Use .jcall rather than $ operator
Add PKI
Added secure to description
Started test harness; other code cleanup

- Started test harness (#3) using a SQL Server I have access to. Details
for server are of course encrypted using the `secure` package. NB: there
seem to be some bugs in some `secure` methods so test harness doesn't
work at the moment
- dbIsValid method for SQLServerConnection returns TRUE when connection
is open rather than FALSE
- Cleaned up further calls to JTDS/SQL methods.