Support SQL tracing

SQLite supports SQL tracing which is truly useful for debugging prepared statements. Similar functionality may be supported by other backends as well.

Tracing works like this:

``` c++
static void profile(void *context, const char *sql)
{
    fprintf(stderr, "Query: %s\n", sql);
}

int main()
{
    ...
    sqlite3_trace(db, profile, NULL);
    ...
}
```
