Update class-wpdb.php to support NVARCHAR

[srutzky]

This fixes #515 .

Add "N" prefix to quoted string substitution when preparing SQL for execution. This will change string literals to be interpreted as NVARCHAR (i.e. Unicode / UTF-16) instead of VARCHAR (i.e. an 8-bit encoding, which changes all characters not in the associated code page to "?"s).

PLEASE NOTE: I don't have time to test this fix. If it helps, there are some notes on what should be tested in PR #422 .