Added Exec implementation
Added test for columns names
Added test for number of rows
Added DB.Close() implementation
Really pass parameters to queries
Added support for string arguments

Adaptation target type changed to *string from []byte because using
[]byte to and from C implies passing to string anyway, thus another copy.

Using *string instead of string to handle nils too.

Go doesn't support encodings, so for the moment we only support utf-8
connections.
Added Tx implementation