Add a new `RegisterBusyHandler` method to `SQLiteConn` to allow access to `sqlite3_busy_handler()`

[polyscone]

This PR just adds a new RegisterBusyHandler method to the SQLiteConn type to allow use of the sqlite3_busy_handler() function.

I tried to follow the same style as the other Register* methods which use the void* user data args as a way to pass a handle to a Go callback function to a trampoline function defined in C.

I've also updated the _examples folder with a new example file.

Fixes #290