Make sure table exists when checking primary key

When asking for a table's primary key column, returning 0 doesn't make
sense if the table doesn't exist in the database. We throw an exception
instead.
Use @Before,@After to handle Statement
Add unit test for tableExists
Test getIntegerPrimaryKey on nonexistant table
Merge pull request #329 from agouge/check-table-exists

Make sure table exists when checking primary key
When asking for a table's primary key column, returning 0 doesn't make
sense if the table doesn't exist in the database. We throw an exception
instead.